[POA-1965] Add truncate function in time span (#218)

Added a new `truncate()` function in time span class, which truncate
both start and end time by the given duration.

time_span/time_span.go

@@ -119,6 +119,14 @@ func (t ClosedInterval) Combine(t2 ClosedInterval) ClosedInterval {
 	}
 }
 
+// Returns a new ClosedInterval with the start and end times truncated.
+func (t ClosedInterval) Truncate(d time.Duration) ClosedInterval {
+	return ClosedInterval{
+		Start: t.Start.Truncate(d),
+		End:   t.End.Truncate(d),
+	}
+}
+
 // An half-open interval [start,end)
 // Empty intervals are represented by having Start >= End.
 type HalfOpenInterval BaseInterval
@@ -205,6 +213,14 @@ func (t HalfOpenInterval) Combine(t2 HalfOpenInterval) HalfOpenInterval {
 	}
 }
 
+// Returns a new ClosedInterval with the start and end times truncated.
+func (t HalfOpenInterval) Truncate(d time.Duration) HalfOpenInterval {
+	return HalfOpenInterval{
+		Start: t.Start.Truncate(d),
+		End:   t.End.Truncate(d),
+	}
+}
+
 type TimeSpan = HalfOpenInterval
 
 func NewTimeSpan(start time.Time, end time.Time) *TimeSpan {