I have an application in Apex where the user selects the start time `F1` of an event and the end time `F2`, and the result should be displayed in a third text field `F3`.

**How can I perform the operation of subtracting `F1` and `F2` using a PL / SQL function?**

For example 7:00 and 7:30 and the result is 30 min.

The time is entered as text, so I use `to_date` to convert the two fields to date.