I am managing SharePoint Online sites for several tenants and a month ago we've suddenly experienced the problem with Date/Time columns. Namely, all the sites have "Serbian (Latin, Serbia)" defined as region locale for representing numbers, date and time. Everything was working fine until time delimiter changed by itself from colon (:) to dot (.) with all tenants on their site collections (they are all different companies).
The problem manifests as follows – first, when the user wants to enter time, it is represented both with colon and a dot:
Then, when he wants to choose the time, it is represented with only a dot in the picker, and that is entered in the field when it is chosen:
However, when the user tries to save the result of the form to the list, the warning appears:
The only thing the user can do is to manually correct the field with colon as time delimiter and SharePoint allows him to save it now. However, when we look at the result in the list itself, it is still represented with single dot as time delimiter:
To repeat, this happens with 6 different tenants that are using the same locale settings, and it appeared with all of them at the same time, with no prior interventions on site collections (I am the sole admin for all of them, using separate accounts within their Office 365 subscriptions) days before.
Taking all of that into account, the only logical explanation to me is that something changed on a global Office 365 scale – please investigate.