For Post-Checkin Offer campaigns, the start date and the end date are on the basis of UTC timezone and we do not have any parameter to change the time zone for this campaign to any other timezone.
These campaigns are based on Receipt time, so if we select a particular day then all the checks with that particular date (irrespective of timezone) will be eligible for these campaigns.
For example, if we are configuring a Post Checkin offer campaign to run on the 14th of March, then all the checks that have a check date as 14th of March will qualify for this campaign.