There is no straight cut solution for this. DateTime is used many different ways for many different reasons in nopCommerce.
Usually DateTime input field takes value fom a user in local time. Then system converts it to UTC time and saves to database. Again when it’s time to show it to the user, system converts that UTC time to local time. These things are managed by DateTimeHelper class.
In your case, you can take input value in persian local time, then convert it to Gregorian local time and finally Gregorian local time to UTC time. You can add methods for these to DateTimeHelper class to make it easier.