Pass date field to button before Save

I have an edit page on a 1 record table. Using it as a place for action buttons and to change transaction date.

I have a date field, in that table, that uses the calendar lookup. Have been trying to have within On Server

$my_transaction_date = $values["transaction_date"];

then use this to update many records with a sql update, but I get blank $my_transaction_date. I am missing something.

My workaround is to have the user press the Edit Save button, then push my custom button.