use correct first month value when not set while truncating to quarter

Author: pjain1

runtime/pkg/timeutil/timeutil.go

@@ -125,6 +125,12 @@ func TruncateTime(tm time.Time, tg TimeGrain, tz *time.Location, firstDay, first
 		tm = tm.In(time.UTC)
 		return tm
 	case TimeGrainQuarter:
+		if firstMonth < 1 {
+			firstMonth = 1
+		}
+		if firstMonth > 12 {
+			firstMonth = 12
+		}
 		monthsToSubtract := (3 + int(tm.Month()) - firstMonth%3) % 3
 		tm = tm.In(tz)
 		tm = time.Date(tm.Year(), tm.Month(), 1, 0, 0, 0, 0, tz)