In MainUI see DateTime Standalone Widget and DateTime List Item for date time selection widgets. Note that there is nothing special about these. They are really just standard configuration of the oh-input widgets.
If you want to use openHAB to drive this see Creating Capabilities with Rule Templates: Time of Day to drive it.
Or for a more comprehensive (but therefore complicated) approach to see Time Based State Machine.
Another fully comprehensive approach is Timeline. This one requires installing some external stuff from GitHub I think to drive the widget.