How can I add time to an existing time selection using a single choice button?

Hello,

I would like to add 1 hour and 45 min on to an existing time selected to calculate when someone should arrive at a venue. 

In this case, the user first selects how many ushers they will need for their event.

Based on the number of ushers, they will need to arrive at the venue between 1hr and 45 min, to 1hr and 20min, prior to the start of the event. 

When the user fills out what time their event will start, I would like a field to be populated with the time that the ushers will need to arrive.

For example, if the user says that their event starts at 3:00 pm, then ushers need to arrive at 1:15 pm.

Is this possible, or has this been answered before? 

I was unable to find the right solution on the support page.

Thank you.

I am checking for a way to update the time conditionally to and hour and 45 minutes before another time but so far I have only been able to copy the values, not update them.  If I find a solution I will update you here in this thread.

Thank you. I managed to find this after quite a bit of sleuthing.

https://www.jotform.com/answers/1166322-How-can-I-subtract-90-minutes-from-the-Time-Field

It is partially what I am looking for, but I if there is a way to get the final calculation to appear in am/pm format, that would be perfect.

But I do not have enough experience with Jotform to figure out how to achieve that.