An easy way to create datetime restriction for many customers with "date-sensitive ads" would be to have them write startdate/enddate in the filename.
Then put this in the dropbox folder. And digital signage would filter accordingly.
This would be much easier than hazzle with scheduling.
For example:
20190625_0600-20190705_1800_-_batmanpremiere.jpg
or just:
20190625-20190705_-_batmanpremiere.jpg
and day restrictions perhaps like:
20190625-20190901_-_goodmorning_0400-0915.jpg
Then customer will only use windows explorer to make the scheduling.