concatenate a date and a time field to one date/time field

I have a start event and stop event that is recorded as separate date and time of day fields. I want to be able to calculate duration but need the date as some events start today but end tomorrow.

I have separate fields to assist in data entry because you can simply type "t" for today in the date field (lovely if there is a shortcut key such as "n" (now) for time - is there?) and then just type the time. In the date/time type field the shortcut "t" doesn't work so you end up having to type MM/DD/YY and HH:MM (and PM if not the default AM).

If you know of a better faster way - let me know!