It is possible what you are entering is an invalid date. Can you give us an example of what you are typing into the field?
What Jaymo said. In addition to example, can you post the script you are using? are there triggers on the field?
Paste a copy of your script too.
If you have FileMaker Advanced, you could enable the debugger before editing the date field to get a better idea of how/why the error happens.
Also, has. Day/month/year always been the date format for your system?
Jaymo Yes clearly in the eyes of FMP it is an invalid date. However it is no different from the date the script has been creating for five years.
ATTN - as I asked the question I went prowling and looked at my system dates and double checked the Short Form [OSX/Date & Time/ advanced] which I had fiddled with to show 22 Mar 17. Reset that to numbers only - 22 03 17 and now all seems to be well. Go figure. There is a Lesson there somewhere.
Thanks for my exploring on your time.
phil see above. ya never know!