Thank you for your post!
You might to be able to have 2nd Field be a Calculation off of the first field. For Example:
1. Create Field1 as a Time Field
2. Create Field2 as a Calculation for Time: Field1 + 1800 (the number is in seconds -30 minutes is 1800 seconds)
3. Input 5:00:00 for Field1
*Field2 displays 5:30:00
I'm also going to move this thread from the FileMaker Community Feedback Space (which is specifically for input on the Community itself) to the Discussions Space where you should receive more views and potentially more feedback on this topic!