Changing datetimepicker to specific times

I have an auction plugin I am using on wordpress for my website and it allows vendors to select their start time and end time. I would like to change the end time so that they are limited to options I set for example they pick the current time but then have to choose the auction running for 30 minutes, 1 hour or 2 hours etc before it ends. Can someone help me with the code pictured to allow this to happen. I am a newbie to coding so bare with me. I hope this is the correct code i have screenshot

It would be much easier for people to help you by putting the actual code in between the code tags </>.

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