Events export as ICS (#4958)

There are a few more pending items. I've added TODO entries for them. The PR was getting pretty big and so I thought I would address them in a followup PR.

---

#### Does this PR need a docs update or release note?

- [ ] ✅ Yes, it's included
- [ ] 🕐 Yes, but in a later PR
- [ ] ⛔ No

#### Type of change

<!--- Please check the type of change your PR introduces: --->
- [ ] 🌻 Feature
- [ ] 🐛 Bugfix
- [ ] 🗺️ Documentation
- [ ] 🤖 Supportability/Tests
- [ ] 💻 CI/Deployment
- [ ] 🧹 Tech Debt/Cleanup

#### Issue(s)

<!-- Can reference multiple issues. Use one of the following "magic words" - "closes, fixes" to auto-close the Github issue. -->
* https://github.com/alcionai/corso/issues/3890

#### Test Plan

<!-- How will this be tested prior to merging.-->
- [ ] 💪 Manual
- [ ] ⚡ Unit test
- [ ] 💚 E2E

src/cmd/converter/converter.go

@@ -7,6 +7,7 @@ import (
 	"os"
 
 	"github.com/alcionai/corso/src/internal/converters/eml"
+	"github.com/alcionai/corso/src/internal/converters/ics"
 	"github.com/alcionai/corso/src/internal/converters/vcf"
 )
 
@@ -40,6 +41,11 @@ func main() {
 			if err != nil {
 				log.Fatal(err)
 			}
+		case "ics":
+			out, err = ics.FromJSON(context.Background(), body)
+			if err != nil {
+				log.Fatal(err)
+			}
 		default:
 			log.Fatal("Unknown target format", to)
 		}

src/go.mod

@@ -59,6 +59,7 @@ require (
 	github.com/VividCortex/ewma v1.2.0 // indirect
 	github.com/acarl005/stripansi v0.0.0-20180116102854-5a71ef0e047d // indirect
 	github.com/andybalholm/brotli v1.0.6 // indirect
+	github.com/arran4/golang-ical v0.2.3 // indirect
 	github.com/aws/aws-sdk-go v1.48.6 // indirect
 	github.com/cention-sany/utf7 v0.0.0-20170124080048-26cad61bd60a // indirect
 	github.com/fsnotify/fsnotify v1.7.0 // indirect

src/go.sum

@@ -33,6 +33,8 @@ github.com/andybalholm/brotli v1.0.6 h1:Yf9fFpf49Zrxb9NlQaluyE92/+X7UVHlhMNJN2sx
 github.com/andybalholm/brotli v1.0.6/go.mod h1:fO7iG3H7G2nSZ7m0zPUDn85XEX2GTukHGRSepvi9Eig=
 github.com/armon/go-metrics v0.4.1 h1:hR91U9KYmb6bLBYLQjyM+3j+rcd/UhE+G78SFnF8gJA=
 github.com/armon/go-metrics v0.4.1/go.mod h1:E6amYzXo6aW1tqzoZGT755KkbgrJsSdpwZ+3JqfkOG4=
+github.com/arran4/golang-ical v0.2.3 h1:C4Vj7+BjJBIrAJhHgi6Ku+XUkQVugRq4re5Cqj5QVdE=
+github.com/arran4/golang-ical v0.2.3/go.mod h1:RqMuPGmwRRwjkb07hmm+JBqcWa1vF1LvVmPtSZN2OhQ=
 github.com/aws/aws-sdk-go v1.48.6 h1:hnL/TE3eRigirDLrdRE9AWE1ALZSVLAsC4wK8TGsMqk=
 github.com/aws/aws-sdk-go v1.48.6/go.mod h1:LF8svs817+Nz+DmiMQKTO3ubZ/6IaTpq3TjupRn3Eqk=
 github.com/aws/aws-xray-sdk-go v1.8.3 h1:S8GdgVncBRhzbNnNUgTPwhEqhwt2alES/9rLASyhxjU=
@@ -57,6 +59,7 @@ github.com/circonus-labs/circonusllhist v0.1.3/go.mod h1:kMXHVDlOchFAehlya5ePtbp
 github.com/cjlapao/common-go v0.0.39 h1:bAAUrj2B9v0kMzbAOhzjSmiyDy+rd56r2sy7oEiQLlA=
 github.com/cjlapao/common-go v0.0.39/go.mod h1:M3dzazLjTjEtZJbbxoA5ZDiGCiHmpwqW9l4UWaddwOA=
 github.com/cpuguy83/go-md2man/v2 v2.0.3/go.mod h1:tgQtvFlXSQOSOSIRvRPT7W67SCa46tRHOmNcaadrF8o=
+github.com/creack/pty v1.1.9/go.mod h1:oKZEueFk5CKHvIhNR5MUki03XCEU+Q6VDXinZuGJ33E=
 github.com/danieljoos/wincred v1.2.0 h1:ozqKHaLK0W/ii4KVbbvluM91W2H3Sh0BncbUNPS7jLE=
 github.com/danieljoos/wincred v1.2.0/go.mod h1:FzQLLMKBFdvu+osBrnFODiv32YGwCfx0SkRa/eYHgec=
 github.com/davecgh/go-spew v1.1.0/go.mod h1:J7Y8YcW2NihsgmVo/mv3lAwl/skON4iLHjSsI+c5H38=
@@ -230,6 +233,7 @@ github.com/natefinch/atomic v1.0.1 h1:ZPYKxkqQOx3KZ+RsbnP/YsgvxWQPGxjC0oBt2AhwV0
 github.com/natefinch/atomic v1.0.1/go.mod h1:N/D/ELrljoqDyT3rZrsUmtsuzvHkeB/wWjHV22AZRbM=
 github.com/nbio/st v0.0.0-20140626010706-e9e8d9816f32 h1:W6apQkHrMkS0Muv8G/TipAy/FJl/rCYT0+EuS8+Z0z4=
 github.com/nbio/st v0.0.0-20140626010706-e9e8d9816f32/go.mod h1:9wM+0iRr9ahx58uYLpLIr5fm8diHn0JbqRycJi6w0Ms=
+github.com/niemeyer/pretty v0.0.0-20200227124842-a10e7caefd8e/go.mod h1:zD1mROLANZcx1PVRCS0qkT7pwLkGfwJo4zjcN/Tysno=
 github.com/olekukonko/tablewriter v0.0.5 h1:P2Ga83D34wi1o9J6Wh1mRuqd4mF/x/lgBS7N7AbDhec=
 github.com/olekukonko/tablewriter v0.0.5/go.mod h1:hPp6KlRPjbx+hW8ykQs1w3UBbZlj6HuIJcUGPhkA7kY=
 github.com/pascaldekloe/goe v0.1.0 h1:cBOtyMzM9HTpWjXfbbunk26uA6nG3a8n06Wieeh0MwY=
@@ -413,6 +417,7 @@ google.golang.org/protobuf v1.31.0/go.mod h1:HV8QOd/L58Z+nl8r43ehVNZIU/HEI6OcFqw
 gopkg.in/alecthomas/kingpin.v2 v2.2.6/go.mod h1:FMv+mEhP44yOT+4EoQTLFTRgOQ1FBLkstjWtayDeSgw=
 gopkg.in/check.v1 v0.0.0-20161208181325-20d25e280405/go.mod h1:Co6ibVJAznAaIkqp8huTwlJQCZ016jof/cbN4VW5Yz0=
 gopkg.in/check.v1 v1.0.0-20190902080502-41f04d3bba15/go.mod h1:Co6ibVJAznAaIkqp8huTwlJQCZ016jof/cbN4VW5Yz0=
+gopkg.in/check.v1 v1.0.0-20200227125254-8fa46927fb4f/go.mod h1:Co6ibVJAznAaIkqp8huTwlJQCZ016jof/cbN4VW5Yz0=
 gopkg.in/check.v1 v1.0.0-20201130134442-10cb98267c6c h1:Hei/4ADfdWqJk1ZMxUNpqntNwaWcugrBjAiHlqqRiVk=
 gopkg.in/check.v1 v1.0.0-20201130134442-10cb98267c6c/go.mod h1:JHkPIbrfpd72SG/EVd6muEfDQjcINNoR0C8j2r3qZ4Q=
 gopkg.in/ini.v1 v1.67.0 h1:Dgnx+6+nfE+IfzjUEISNeydPJh9AXNNsWbGP9KzCsOA=
@@ -425,6 +430,7 @@ gopkg.in/yaml.v2 v2.2.8/go.mod h1:hI93XBmqTisBFMUTm0b8Fm+jr3Dg1NNxqwp+5A1VGuI=
 gopkg.in/yaml.v2 v2.4.0 h1:D8xgwECY7CYvx+Y2n4sBz93Jn9JRvxdiyyo8CTfuKaY=
 gopkg.in/yaml.v2 v2.4.0/go.mod h1:RDklbk79AGWmwhnvt/jBztapEOGDOx6ZbXqjP6csGnQ=
 gopkg.in/yaml.v3 v3.0.0-20200313102051-9f266ea9e77c/go.mod h1:K4uyk7z7BCEPqu6E+C64Yfv1cQ7kz7rIZviUmN+EgEM=
+gopkg.in/yaml.v3 v3.0.0/go.mod h1:K4uyk7z7BCEPqu6E+C64Yfv1cQ7kz7rIZviUmN+EgEM=
 gopkg.in/yaml.v3 v3.0.1 h1:fxVm/GzAzEWqLHuvctI91KS9hhNmmWOoWu0XTYJS7CA=
 gopkg.in/yaml.v3 v3.0.1/go.mod h1:K4uyk7z7BCEPqu6E+C64Yfv1cQ7kz7rIZviUmN+EgEM=
 gotest.tools/v3 v3.5.1 h1:EENdUnS3pdur5nybKYIh2Vfgc8IUNBjxDPSjtiJcOzU=

src/internal/converters/ics/consts.go

@@ -0,0 +1,168 @@
+package ics
+
+var (
+	// Map from Graph API recurrence type to iCal recurrence type
+	GraphToICalIndex = map[string]int{
+		"first":  1,
+		"second": 2,
+		"third":  3,
+		"fourth": 4,
+		"last":   -1,
+	}
+
+	// Map from Graph API day of week representation to iCal day of week representation
+	GraphToICalDOW = map[string]string{
+		"sunday":    "SU",
+		"monday":    "MO",
+		"tuesday":   "TU",
+		"wednesday": "WE",
+		"thursday":  "TH",
+		"friday":    "FR",
+		"saturday":  "SA",
+	}
+)
+
+// Map from Window time zone to TZ database time zone
+// https://github.com/closeio/sync-engine/blob/1ce0e1ad0104a2ab2479da09b073c86f4feee5f9/inbox/events/timezones.py#L6
+var GraphTimeZoneToTZ = map[string]string{
+	"AUS Central Standard Time":       "Australia/Darwin",
+	"AUS Eastern Standard Time":       "Australia/Sydney",
+	"Afghanistan Standard Time":       "Asia/Kabul",
+	"Alaskan Standard Time":           "America/Anchorage",
+	"Aleutian Standard Time":          "America/Adak",
+	"Altai Standard Time":             "Asia/Barnaul",
+	"Arab Standard Time":              "Asia/Riyadh",
+	"Arabian Standard Time":           "Asia/Dubai",
+	"Arabic Standard Time":            "Asia/Baghdad",
+	"Argentina Standard Time":         "America/Buenos_Aires",
+	"Astrakhan Standard Time":         "Europe/Astrakhan",
+	"Atlantic Standard Time":          "America/Halifax",
+	"Aus Central W. Standard Time":    "Australia/Eucla",
+	"Azerbaijan Standard Time":        "Asia/Baku",
+	"Azores Standard Time":            "Atlantic/Azores",
+	"Bahia Standard Time":             "America/Bahia",
+	"Bangladesh Standard Time":        "Asia/Dhaka",
+	"Belarus Standard Time":           "Europe/Minsk",
+	"Bougainville Standard Time":      "Pacific/Bougainville",
+	"Canada Central Standard Time":    "America/Regina",
+	"Cape Verde Standard Time":        "Atlantic/Cape_Verde",
+	"Caucasus Standard Time":          "Asia/Yerevan",
+	"Cen. Australia Standard Time":    "Australia/Adelaide",
+	"Central America Standard Time":   "America/Guatemala",
+	"Central Asia Standard Time":      "Asia/Almaty",
+	"Central Brazilian Standard Time": "America/Cuiaba",
+	"Central Europe Standard Time":    "Europe/Budapest",
+	"Central European Standard Time":  "Europe/Warsaw",
+	"Central Pacific Standard Time":   "Pacific/Guadalcanal",
+	"Central Standard Time":           "America/Chicago",
+	"Central Standard Time (Mexico)":  "America/Mexico_City",
+	"Chatham Islands Standard Time":   "Pacific/Chatham",
+	"China Standard Time":             "Asia/Shanghai",
+	"Cuba Standard Time":              "America/Havana",
+	"Dateline Standard Time":          "Etc/GMT+12",
+	"E. Africa Standard Time":         "Africa/Nairobi",
+	"E. Australia Standard Time":      "Australia/Brisbane",
+	"E. Europe Standard Time":         "Europe/Chisinau",
+	"E. South America Standard Time":  "America/Sao_Paulo",
+	"Easter Island Standard Time":     "Pacific/Easter",
+	"Eastern Standard Time":           "America/New_York",
+	"Eastern Standard Time (Mexico)":  "America/Cancun",
+	"Egypt Standard Time":             "Africa/Cairo",
+	"Ekaterinburg Standard Time":      "Asia/Yekaterinburg",
+	"FLE Standard Time":               "Europe/Kiev",
+	"Fiji Standard Time":              "Pacific/Fiji",
+	"GMT Standard Time":               "Europe/London",
+	"GTB Standard Time":               "Europe/Bucharest",
+	"Georgian Standard Time":          "Asia/Tbilisi",
+	"Greenland Standard Time":         "America/Godthab",
+	"Greenwich Standard Time":         "Atlantic/Reykjavik",
+	"Haiti Standard Time":             "America/Port-au-Prince",
+	"Hawaiian Standard Time":          "Pacific/Honolulu",
+	"India Standard Time":             "Asia/Calcutta",
+	"Iran Standard Time":              "Asia/Tehran",
+	"Israel Standard Time":            "Asia/Jerusalem",
+	"Jordan Standard Time":            "Asia/Amman",
+	"Kaliningrad Standard Time":       "Europe/Kaliningrad",
+	"Korea Standard Time":             "Asia/Seoul",
+	"Libya Standard Time":             "Africa/Tripoli",
+	"Line Islands Standard Time":      "Pacific/Kiritimati",
+	"Lord Howe Standard Time":         "Australia/Lord_Howe",
+	"Magadan Standard Time":           "Asia/Magadan",
+	"Magallanes Standard Time":        "America/Punta_Arenas",
+	"Marquesas Standard Time":         "Pacific/Marquesas",
+	"Mauritius Standard Time":         "Indian/Mauritius",
+	"Middle East Standard Time":       "Asia/Beirut",
+	"Montevideo Standard Time":        "America/Montevideo",
+	"Morocco Standard Time":           "Africa/Casablanca",
+	"Mountain Standard Time":          "America/Denver",
+	"Mountain Standard Time (Mexico)": "America/Chihuahua",
+	"Myanmar Standard Time":           "Asia/Rangoon",
+	"N. Central Asia Standard Time":   "Asia/Novosibirsk",
+	"Namibia Standard Time":           "Africa/Windhoek",
+	"Nepal Standard Time":             "Asia/Katmandu",
+	"New Zealand Standard Time":       "Pacific/Auckland",
+	"Newfoundland Standard Time":      "America/St_Johns",
+	"Norfolk Standard Time":           "Pacific/Norfolk",
+	"North Asia East Standard Time":   "Asia/Irkutsk",
+	"North Asia Standard Time":        "Asia/Krasnoyarsk",
+	"North Korea Standard Time":       "Asia/Pyongyang",
+	"Omsk Standard Time":              "Asia/Omsk",
+	"Pacific SA Standard Time":        "America/Santiago",
+	"Pacific Standard Time":           "America/Los_Angeles",
+	"Pacific Standard Time (Mexico)":  "America/Tijuana",
+	"Pakistan Standard Time":          "Asia/Karachi",
+	"Paraguay Standard Time":          "America/Asuncion",
+	"Qyzylorda Standard Time":         "Asia/Qyzylorda",
+	"Romance Standard Time":           "Europe/Paris",
+	"Russia Time Zone 10":             "Asia/Srednekolymsk",
+	"Russia Time Zone 11":             "Asia/Kamchatka",
+	"Russia Time Zone 3":              "Europe/Samara",
+	"Russian Standard Time":           "Europe/Moscow",
+	"SA Eastern Standard Time":        "America/Cayenne",
+	"SA Pacific Standard Time":        "America/Bogota",
+	"SA Western Standard Time":        "America/La_Paz",
+	"SE Asia Standard Time":           "Asia/Bangkok",
+	"Saint Pierre Standard Time":      "America/Miquelon",
+	"Sakhalin Standard Time":          "Asia/Sakhalin",
+	"Samoa Standard Time":             "Pacific/Apia",
+	"Sao Tome Standard Time":          "Africa/Sao_Tome",
+	"Saratov Standard Time":           "Europe/Saratov",
+	"Singapore Standard Time":         "Asia/Singapore",
+	"South Africa Standard Time":      "Africa/Johannesburg",
+	"South Sudan Standard Time":       "Africa/Juba",
+	"Sri Lanka Standard Time":         "Asia/Colombo",
+	"Sudan Standard Time":             "Africa/Khartoum",
+	"Syria Standard Time":             "Asia/Damascus",
+	"Taipei Standard Time":            "Asia/Taipei",
+	"Tasmania Standard Time":          "Australia/Hobart",
+	"Tocantins Standard Time":         "America/Araguaina",
+	"Tokyo Standard Time":             "Asia/Tokyo",
+	"Tomsk Standard Time":             "Asia/Tomsk",
+	"Tonga Standard Time":             "Pacific/Tongatapu",
+	"Transbaikal Standard Time":       "Asia/Chita",
+	"Turkey Standard Time":            "Europe/Istanbul",
+	"Turks And Caicos Standard Time":  "America/Grand_Turk",
+	"US Eastern Standard Time":        "America/Indianapolis",
+	"US Mountain Standard Time":       "America/Phoenix",
+	"UTC":                             "Etc/UTC",
+	"UTC+12":                          "Etc/GMT-12",
+	"UTC+13":                          "Etc/GMT-13",
+	"UTC-02":                          "Etc/GMT+2",
+	"UTC-08":                          "Etc/GMT+8",
+	"UTC-09":                          "Etc/GMT+9",
+	"UTC-11":                          "Etc/GMT+11",
+	"Ulaanbaatar Standard Time":       "Asia/Ulaanbaatar",
+	"Venezuela Standard Time":         "America/Caracas",
+	"Vladivostok Standard Time":       "Asia/Vladivostok",
+	"Volgograd Standard Time":         "Europe/Volgograd",
+	"W. Australia Standard Time":      "Australia/Perth",
+	"W. Central Africa Standard Time": "Africa/Lagos",
+	"W. Europe Standard Time":         "Europe/Berlin",
+	"W. Mongolia Standard Time":       "Asia/Hovd",
+	"West Asia Standard Time":         "Asia/Tashkent",
+	"West Bank Standard Time":         "Asia/Hebron",
+	"West Pacific Standard Time":      "Pacific/Port_Moresby",
+	"Yakutsk Standard Time":           "Asia/Yakutsk",
+	"Yukon Standard Time":             "America/Whitehorse",
+	"tzone://Microsoft/Utc":           "Etc/UTC",
+}

src/internal/converters/ics/ics.go

@@ -0,0 +1,447 @@
+package ics
+
+import (
+	"context"
+	"encoding/base64"
+	"fmt"
+	"strings"
+	"time"
+
+	"github.com/alcionai/clues"
+	ics "github.com/arran4/golang-ical"
+	"github.com/microsoftgraph/msgraph-sdk-go/models"
+
+	"github.com/alcionai/corso/src/internal/common/ptr"
+	"github.com/alcionai/corso/src/internal/common/str"
+	"github.com/alcionai/corso/src/pkg/dttm"
+	"github.com/alcionai/corso/src/pkg/services/m365/api"
+)
+
+// This package is used to convert json response from graph to ics
+// Ref: https://icalendar.org/
+// Ref: https://www.rfc-editor.org/rfc/rfc5545
+// Ref: https://learn.microsoft.com/en-us/graph/api/resources/event?view=graph-rest-1.0
+
+// TODO: Items not handled
+// locations (different from location)
+// exceptions and modifications
+
+// Field in the backed up data that we cannot handle
+// allowNewTimeProposals, hideAttendees, importance, isOnlineMeeting,
+// isOrganizer, isReminderOn, onlineMeeting, onlineMeetingProvider,
+// onlineMeetingUrl, originalEndTimeZone, originalStart,
+// originalStartTimeZone, reminderMinutesBeforeStart, responseRequested,
+// responseStatus, sensitivity
+
+func keyValues(key, value string) *ics.KeyValues {
+	return &ics.KeyValues{
+		Key:   key,
+		Value: []string{value},
+	}
+}
+
+func getLocationString(location models.Locationable) string {
+	if location == nil {
+		return ""
+	}
+
+	dn := ptr.Val(location.GetDisplayName())
+	segments := []string{dn}
+
+	// TODO: Handle different location types
+	addr := location.GetAddress()
+	if addr != nil {
+		street := ptr.Val(addr.GetStreet())
+		city := ptr.Val(addr.GetCity())
+		state := ptr.Val(addr.GetState())
+		country := ptr.Val(addr.GetCountryOrRegion())
+		postal := ptr.Val(addr.GetPostalCode())
+
+		segments = append(segments, street, city, state, country, postal)
+	}
+
+	nonEmpty := []string{}
+
+	for _, seg := range segments {
+		if len(seg) > 0 {
+			nonEmpty = append(nonEmpty, seg)
+		}
+	}
+
+	return strings.Join(nonEmpty, ", ")
+}
+
+func getUTCTime(ts, tz string) (time.Time, error) {
+	// Timezone is always converted to UTC.  This is the easiest way to
+	// ensure we have the correct time as the .ics file expects the same
+	// timezone everywhere according to the spec.
+	it, err := dttm.ParseTime(ts)
+	if err != nil {
+		return time.Now(), clues.Wrap(err, "parsing time")
+	}
+
+	timezone, ok := GraphTimeZoneToTZ[tz]
+	if !ok {
+		return it, clues.New("unknown timezone")
+	}
+
+	loc, err := time.LoadLocation(timezone)
+	if err != nil {
+		return time.Now(), clues.Wrap(err, "loading timezone")
+	}
+
+	// embed timezone
+	locTime := time.Date(it.Year(), it.Month(), it.Day(), it.Hour(), it.Minute(), it.Second(), 0, loc)
+
+	return locTime.UTC(), nil
+}
+
+// https://www.rfc-editor.org/rfc/rfc5545#section-3.8.5.3
+// https://www.rfc-editor.org/rfc/rfc5545#section-3.3.10
+// https://learn.microsoft.com/en-us/graph/api/resources/patternedrecurrence?view=graph-rest-1.0
+// Ref: https://github.com/closeio/sync-engine/pull/381/files
+func getRecurrencePattern(
+	ctx context.Context,
+	recurrence models.PatternedRecurrenceable,
+) (string, error) {
+	recurComponents := []string{}
+	pat := recurrence.GetPattern()
+
+	freq := pat.GetTypeEscaped()
+	if freq != nil {
+		switch *freq {
+		case models.DAILY_RECURRENCEPATTERNTYPE:
+			recurComponents = append(recurComponents, "FREQ=DAILY")
+		case models.WEEKLY_RECURRENCEPATTERNTYPE:
+			recurComponents = append(recurComponents, "FREQ=WEEKLY")
+		case models.ABSOLUTEMONTHLY_RECURRENCEPATTERNTYPE, models.RELATIVEMONTHLY_RECURRENCEPATTERNTYPE:
+			recurComponents = append(recurComponents, "FREQ=MONTHLY")
+		case models.ABSOLUTEYEARLY_RECURRENCEPATTERNTYPE, models.RELATIVEYEARLY_RECURRENCEPATTERNTYPE:
+			recurComponents = append(recurComponents, "FREQ=YEARLY")
+		}
+	}
+
+	interval := pat.GetInterval()
+	if interval != nil {
+		recurComponents = append(recurComponents, "INTERVAL="+fmt.Sprint(ptr.Val(interval)))
+	}
+
+	month := ptr.Val(pat.GetMonth())
+	if month > 0 {
+		recurComponents = append(recurComponents, "BYMONTH="+fmt.Sprint(month))
+	}
+
+	// This is required if absoluteMonthly or absoluteYearly
+	day := ptr.Val(pat.GetDayOfMonth())
+	if day > 0 {
+		recurComponents = append(recurComponents, "BYMONTHDAY="+fmt.Sprint(day))
+	}
+
+	dow := pat.GetDaysOfWeek()
+	if dow != nil {
+		dowComponents := []string{}
+
+		for _, day := range dow {
+			icalday, ok := GraphToICalDOW[day.String()]
+			if !ok {
+				return "", clues.NewWC(ctx, "unknown day of week").With("day", day.String())
+			}
+
+			dowComponents = append(dowComponents, icalday)
+		}
+
+		index := pat.GetIndex()
+		prefix := ""
+
+		if index != nil &&
+			(ptr.Val(freq) == models.RELATIVEMONTHLY_RECURRENCEPATTERNTYPE ||
+				ptr.Val(freq) == models.RELATIVEYEARLY_RECURRENCEPATTERNTYPE) {
+			prefix = fmt.Sprint(GraphToICalIndex[index.String()])
+		}
+
+		recurComponents = append(recurComponents, "BYDAY="+prefix+strings.Join(dowComponents, ","))
+	}
+
+	rrange := recurrence.GetRangeEscaped()
+	if rrange != nil {
+		switch ptr.Val(rrange.GetTypeEscaped()) {
+		case models.ENDDATE_RECURRENCERANGETYPE:
+			end := rrange.GetEndDate()
+			if end != nil {
+				// NOTE: We convert just a date into date+time in a
+				// different timezone which will cause it to not be just
+				// a date anymore.
+				endTime, err := getUTCTime(end.String(), ptr.Val(rrange.GetRecurrenceTimeZone()))
+				if err != nil {
+					return "", clues.Wrap(err, "parsing end time")
+				}
+
+				recurComponents = append(recurComponents, "UNTIL="+endTime.Format("20060102T150405Z"))
+			}
+		case models.NOEND_RECURRENCERANGETYPE:
+			// Nothing to do
+		case models.NUMBERED_RECURRENCERANGETYPE:
+			count := ptr.Val(rrange.GetNumberOfOccurrences())
+			if count > 0 {
+				recurComponents = append(recurComponents, "COUNT="+fmt.Sprint(count))
+			}
+		}
+	}
+
+	return strings.Join(recurComponents, ";"), nil
+}
+
+func FromJSON(ctx context.Context, body []byte) (string, error) {
+	data, err := api.BytesToEventable(body)
+	if err != nil {
+		return "", clues.Wrap(err, "converting to eventable")
+	}
+
+	cal := ics.NewCalendar()
+	cal.SetProductId("-//Alcion//Corso") // Does this have to be customizable?
+
+	id := data.GetId() // XXX: iCalUId?
+	event := cal.AddEvent(ptr.Val(id))
+
+	created := data.GetCreatedDateTime()
+	if created != nil {
+		event.SetCreatedTime(ptr.Val(created))
+	}
+
+	modified := data.GetLastModifiedDateTime()
+	if modified != nil {
+		event.SetModifiedAt(ptr.Val(modified))
+	}
+
+	allDay := ptr.Val(data.GetIsAllDay())
+
+	startString := data.GetStart().GetDateTime()
+	timeZone := data.GetStart().GetTimeZone()
+
+	if startString != nil {
+		start, err := getUTCTime(ptr.Val(startString), ptr.Val(timeZone))
+		if err != nil {
+			return "", clues.Wrap(err, "parsing start time")
+		}
+
+		if allDay {
+			event.SetStartAt(start, ics.WithValue(string(ics.ValueDataTypeDate)))
+		} else {
+			event.SetStartAt(start)
+		}
+	}
+
+	endString := data.GetEnd().GetDateTime()
+	timeZone = data.GetEnd().GetTimeZone()
+
+	if endString != nil {
+		end, err := getUTCTime(ptr.Val(endString), ptr.Val(timeZone))
+		if err != nil {
+			return "", clues.Wrap(err, "parsing end time")
+		}
+
+		if allDay {
+			event.SetEndAt(end, ics.WithValue(string(ics.ValueDataTypeDate)))
+		} else {
+			event.SetEndAt(end)
+		}
+	}
+
+	recurrence := data.GetRecurrence()
+	if recurrence != nil {
+		pattern, err := getRecurrencePattern(ctx, recurrence)
+		if err != nil {
+			return "", clues.WrapWC(ctx, err, "generating RRULE")
+		}
+
+		event.AddRrule(pattern)
+	}
+
+	cancelled := data.GetIsCancelled()
+	if cancelled != nil {
+		event.SetStatus(ics.ObjectStatusCancelled)
+	}
+
+	draft := data.GetIsDraft()
+	if draft != nil {
+		event.SetStatus(ics.ObjectStatusDraft)
+	}
+
+	summary := data.GetSubject()
+	if summary != nil {
+		event.SetSummary(ptr.Val(summary))
+	}
+
+	// TODO: Emojies currently don't seem to be read properly by Outlook
+	bodyPreview := ptr.Val(data.GetBodyPreview())
+
+	if data.GetBody() != nil {
+		description := ptr.Val(data.GetBody().GetContent())
+		contentType := data.GetBody().GetContentType().String()
+
+		if len(description) > 0 && contentType == "text" {
+			event.SetDescription(description)
+		} else {
+			// https://stackoverflow.com/a/859475
+			event.SetDescription(bodyPreview)
+
+			if contentType == "html" {
+				desc := strings.ReplaceAll(description, "\r\n", "")
+				desc = strings.ReplaceAll(desc, "\n", "")
+				event.AddProperty("X-ALT-DESC", desc, ics.WithFmtType("text/html"))
+			}
+		}
+	}
+
+	showAs := ptr.Val(data.GetShowAs()).String()
+	if len(showAs) > 0 && showAs != "unknown" {
+		var status ics.FreeBusyTimeType
+
+		switch showAs {
+		case "free":
+			status = ics.FreeBusyTimeTypeFree
+		case "tentative":
+			status = ics.FreeBusyTimeTypeBusyTentative
+		case "busy":
+			status = ics.FreeBusyTimeTypeBusy
+		case "oof", "workingElsewhere": // this is just best effort conversion
+			status = ics.FreeBusyTimeTypeBusyUnavailable
+		}
+
+		event.AddProperty(ics.ComponentPropertyFreebusy, string(status))
+	}
+
+	categories := data.GetCategories()
+	for _, category := range categories {
+		event.AddProperty(ics.ComponentPropertyCategories, category)
+	}
+
+	// According to the RFC, this property may be used in a calendar
+	// component to convey a location where a more dynamic rendition of
+	// the calendar information associated with the calendar component
+	// can be found.
+	url := ptr.Val(data.GetWebLink())
+	if len(url) > 0 {
+		event.SetURL(url)
+	}
+
+	organizer := data.GetOrganizer()
+	if organizer != nil {
+		name := ptr.Val(organizer.GetEmailAddress().GetName())
+		addr := ptr.Val(organizer.GetEmailAddress().GetAddress())
+
+		// TODO: What to do if we only have a name?
+		if len(name) > 0 && len(addr) > 0 {
+			event.SetOrganizer(addr, ics.WithCN(name))
+		} else if len(addr) > 0 {
+			event.SetOrganizer(addr)
+		}
+	}
+
+	attendees := data.GetAttendees()
+	for _, attendee := range attendees {
+		props := []ics.PropertyParameter{}
+
+		atype := attendee.GetTypeEscaped()
+		if atype != nil {
+			var role ics.ParticipationRole
+
+			switch atype.String() {
+			case "required":
+				role = ics.ParticipationRoleReqParticipant
+			case "optional":
+				role = ics.ParticipationRoleOptParticipant
+			case "resource":
+				role = ics.ParticipationRoleNonParticipant
+			}
+
+			props = append(props, keyValues(string(ics.ParameterRole), string(role)))
+		}
+
+		name := ptr.Val(attendee.GetEmailAddress().GetName())
+		if len(name) > 0 {
+			props = append(props, ics.WithCN(name))
+		}
+
+		// Time when a resp change occurred is not recorded
+		if attendee.GetStatus() != nil {
+			resp := ptr.Val(attendee.GetStatus().GetResponse()).String()
+			if len(resp) > 0 && resp != "none" {
+				var pstat ics.ParticipationStatus
+
+				switch resp {
+				case "accepted", "organizer":
+					pstat = ics.ParticipationStatusAccepted
+				case "declined":
+					pstat = ics.ParticipationStatusDeclined
+				case "tentativelyAccepted":
+					pstat = ics.ParticipationStatusTentative
+				case "notResponded":
+					pstat = ics.ParticipationStatusNeedsAction
+				}
+
+				props = append(props, keyValues(string(ics.ParameterParticipationStatus), string(pstat)))
+			}
+		}
+
+		addr := ptr.Val(attendee.GetEmailAddress().GetAddress())
+		event.AddAttendee(addr, props...)
+	}
+
+	location := getLocationString(data.GetLocation())
+	if len(location) > 0 {
+		event.SetLocation(location)
+	}
+
+	// TODO Handle different attachment type (file, item and reference)
+	attachments := data.GetAttachments()
+	for _, attachment := range attachments {
+		props := []ics.PropertyParameter{}
+		contentType := ptr.Val(attachment.GetContentType())
+		name := ptr.Val(attachment.GetName())
+
+		if len(name) > 0 {
+			// FILENAME does not seem to be parsed by Outlook
+			props = append(props,
+				&ics.KeyValues{
+					Key:   "FILENAME",
+					Value: []string{name},
+				})
+		}
+
+		cb, err := attachment.GetBackingStore().Get("contentBytes")
+		if err != nil {
+			return "", clues.Wrap(err, "getting attachment content")
+		}
+
+		content, ok := cb.([]uint8)
+		if !ok {
+			return "", clues.NewWC(ctx, "getting attachment content string")
+		}
+
+		props = append(props, ics.WithEncoding("base64"), ics.WithValue("BINARY"))
+		if len(contentType) > 0 {
+			props = append(props, ics.WithFmtType(contentType))
+		}
+
+		// TODO: Inline attachments don't show up in Outlook
+		inline := ptr.Val(attachment.GetIsInline())
+		if inline {
+			cidv, err := attachment.GetBackingStore().Get("contentId")
+			if err != nil {
+				return "", clues.Wrap(err, "getting attachment content id")
+			}
+
+			cid, err := str.AnyToString(cidv)
+			if err != nil {
+				return "", clues.Wrap(err, "getting attachment content id string")
+			}
+
+			props = append(props, keyValues("CID", cid))
+		}
+
+		event.AddAttachment(base64.StdEncoding.EncodeToString(content), props...)
+	}
+
+	return cal.Serialize(), nil
+}

src/internal/m365/collection/exchange/export.go

@@ -8,6 +8,7 @@ import (
 	"github.com/alcionai/clues"
 
 	"github.com/alcionai/corso/src/internal/converters/eml"
+	"github.com/alcionai/corso/src/internal/converters/ics"
 	"github.com/alcionai/corso/src/internal/converters/vcf"
 	"github.com/alcionai/corso/src/internal/data"
 	"github.com/alcionai/corso/src/pkg/control"
@@ -48,12 +49,16 @@ func streamItems(
 
 	for _, rc := range drc {
 		ictx := clues.Add(ctx, "path_short_ref", rc.FullPath().ShortRef())
-
-		ext := ".eml"
 		category := rc.FullPath().Category()
+		ext := ""
 
-		if category == path.ContactsCategory {
+		switch category {
+		case path.EmailCategory:
+			ext = ".eml"
+		case path.ContactsCategory:
 			ext = ".vcf"
+		case path.EventsCategory:
+			ext = ".ics"
 		}
 
 		for item := range rc.Items(ictx, errs) {
@@ -111,6 +116,20 @@ func streamItems(
 						Error: err,
 					}
 
+					continue
+				}
+			case path.EventsCategory:
+				outData, err = ics.FromJSON(ctx, content)
+				if err != nil {
+					err = clues.Wrap(err, "converting to ics")
+
+					logger.CtxErr(ctx, err).Info("processing collection item")
+
+					ch <- export.Item{
+						ID:    id,
+						Error: err,
+					}
+
 					continue
 				}
 			}

src/pkg/dttm/dttm.go

@@ -69,6 +69,7 @@ var (
 		SafeForTesting,
 		HumanReadable,
 		HumanReadableDriveItem,
+		M365DateTimeTimeZone,
 		Legacy,
 		TabularOutput,
 		ClippedHuman,