Allow for timezone from TZ database (#5078)

<!-- PR description-->

Graph sometimes just send timezone value which are available in TZ
database. This PR makes sure that we don't always try to convert, but
check if the timezone is already in the format that we need first.

---

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

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

#### Type of change

<!--- Please check the type of change your PR introduces: --->
- [ ] 🌻 Feature
- [x] 🐛 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. -->
* #<issue>

#### Test Plan

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

---------

Co-authored-by: zackrossman <zrossman@alcion.ai>

src/internal/converters/ics/ics.go

@@ -72,6 +72,11 @@ func getLocationString(location models.Locationable) string {
 }
 
 func GetUTCTime(ts, tz string) (time.Time, error) {
+	var (
+		loc *time.Location
+		err 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.
@@ -80,15 +85,18 @@ func GetUTCTime(ts, tz string) (time.Time, error) {
 		return time.Time{}, clues.Wrap(err, "parsing time").With("given_time_string", ts)
 	}
 
-	timezone, ok := GraphTimeZoneToTZ[tz]
-	if !ok {
-		return it, clues.New("unknown timezone").With("timezone", tz)
-	}
-
-	loc, err := time.LoadLocation(timezone)
+	loc, err = time.LoadLocation(tz)
 	if err != nil {
-		return time.Time{}, clues.Wrap(err, "loading timezone").
-			With("converted_timezone", timezone)
+		timezone, ok := GraphTimeZoneToTZ[tz]
+		if !ok {
+			return it, clues.New("unknown timezone").With("timezone", tz)
+		}
+
+		loc, err = time.LoadLocation(timezone)
+		if err != nil {
+			return time.Time{}, clues.Wrap(err, "loading timezone").
+				With("converted_timezone", timezone)
+		}
 	}
 
 	// embed timezone

src/internal/converters/ics/ics_test.go

@@ -138,6 +138,13 @@ func (suite *ICSUnitSuite) TestGetUTCTime() {
 			time:      time.Date(2021, 1, 1, 6, 30, 0, 0, time.UTC),
 			errCheck:  require.NoError,
 		},
+		{
+			name:      "timezone from TZ database",
+			timestamp: "2021-01-01T12:00:00Z",
+			timezone:  "America/Los_Angeles",
+			time:      time.Date(2021, 1, 1, 20, 0, 0, 0, time.UTC),
+			errCheck:  require.NoError,
+		},
 		{
 			name:      "invalid time",
 			timestamp: "invalid",