Want to add a custom field that displays both 12 hours and 24 hours time format option to choose.

On prem CRM 2015, I want to add a date and time field on the form customized entity, where a user can/should have an option to choose either 12 hours or 24 hours time format. later based on selection the time on the field should changed. I'm not having user behavioral option, as it's not an updated version.