Server 13 (at least) / Windows.
While setting-up (editing, actually) a server-scheduled FileMaker script performed daily, I was attempting to add an End Date. After entering an end date that falls more than two months after the start date, the FMSAC refused to validate the date (red ! and the indication that "the start date must be before the end date," which of course, it was. But the 'next' button would not permit me to commit my change.
Insanity is the act of repeating something that doesn't work and expecting a different outcome....
Nonetheless, I tried a couple more times, and did finally discover how to circumvent this roadblock:
1. Edit the script
2. Check 'Set the end date."
3. Modify the end date
4. Un-check 'Set the end date'
5. Finish editing (don't cancel, even though you may not have made any changes).
6. Edit the script again
7. Check 'Set the end date' and enter the date again
The date is now validated and 'next' allows you to save the schedule with the end date.