You can't override the validation for a date field. But then, do you just want a calculated display, or does it have to be a date field?
If the former, you can use an auto-enter calc with replace; if the latter, you need a script that is triggered OnValidation, like
Set Field [ YourTable::yourField ; Let ( f = YourTable::yourField ; Replace ( f ; Length ( f ) - 4 ; 0 ; "1/" ) ) ]
Format the date field to only display month name and year.
One solution would be to have two fields, month and year, and then auto-replace the month using a calc to substitute: 1 > January, … 12 > December.
You can set the field up to just be a full standard (global) date field.
Though you enter a full date, FileMaker allows you to control the format of how that date displays.
All the date fields below are the same field. The inspector has been used to control the display of the date.
DateDisplay.fmp12.zip 97.8 K
I played with the idea that you can format your date field with a custom date format:
Then, to speed up data entry, I used an OnObjectKeystroke Trigger to add the day automatically (day 1). So, typing "2/" will immediately become "2/1/". Now, effectively, the user types "2/16" and the date will become 2/1/2016 upon leaving the field; and it will show as above if the field is formatted that way..
Here's the script behind the script trigger: