Many forms have Start, Duration and Stop fields that are interlinked.

If you change start or stop date or time the duration will be recalculated. If you change duration the stop date and time will be updated. The plus and minus buttons next to duration can be set used to increase or decrease duration.

By default 10 minutes increments or decrements are used. By default the duration is set to 1 hour. Both values can be configured on the server.