I have a logic challenge

I have a form that populates an email message using the $_POST.variable. What I want to do is the following:

1. Select a time for a meeting from a dropdown.
   The meeting timeframe is choice of 8:00 AM, 9:00 AM, 1:30 PM, 2:00 PM.
2. Click a date picker and select a date for the meeting.
3. The next date will be for the hotel reservation. If the chosen timeframe is 8:00 AM or 9:00 AM, the
   date for the reservation should be one day earlier than the date of the meeting. If the chosen
   timeframe is later, i.e., 1:30 PM or 2:00 PM, the reservation should be the same date as meeting.

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Any suggestions or help will be very appreciated.