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We also encountered this problem just recently with a customer of ours.
Didn't know exactly the time it started, but we just where informed of the issue.
After reading some posts on the internet, with some possible solutions, nothing seemed to work,
Created a ticket via our partner portal with Microsoft.
After some e-mail back and forth, a support employee watched what was going on.
Basically what was the error on our side (and site):
We had our Time Zone setting on: UTC +1 (Amsterdam) time,
With a Dutch Regional Layout.
Changing the Time Zone to: UTC -8 Pacific (US and Canada)
Made the field work as expected. (Had to Force Reload browser in the list ctrl+F5)
Keeping this setting and it works, changing it back and it stops working.
This will mean every date will be off by 9 hours in our case, dropping a 'day' on our current fields..
No Full solution yet, just got of the phone..