Flight plan departure time for delayed flights

Today, WS670 was scheduled to depart YYC at 13:30 but the departure has been delayed, now departing 16:30. But, when the flight filing was distributed, it still has the 13:30 departure time.

That’s confusing to a lay person like myself. Here I was, thinking that we had hours to go before having to head to the airport, only to get the impression that the delay somehow wasn’t happening and we’d already missed our flight.

So, why is the original departure time still atop the flight plan?