Am I correct that your "form" displays multiple records at the same time?
Even though the field is located in the header, you are entering the date into that field for just one record. In Browse Mode, that field will display its value from the current record so you enter a date and then click on a different record to make it the current record and the field now displays data from the new current record and your date disappears or changes.
In preview mode or when printing/saving as PDF, that same header field will display data from the record positioned at the top of each page so you can find the data changing with each page of your preview or print out.
You can define this field to have global storage (this is a field option specified in manage | Database | Fields) and then it will store one value for all the records in your table and then you will not have this issue.
Thank you so much! That did it!