extremely strange behavior of date/time columns in modern experience

[Chad_V_Kealey]

A user brought this issue to my attention and at first I assumed it was some kind of user error. However, after extensive testing across different sites with different lists, it's clear that there is some kind of bug with the new "modern" date/time control in SharePoint Online.

What is happening is that when the time portion of a date/time column is set to PM (e.g.: 1:00 PM) AND either the month OR the day is a single digit value (e.g.: 01 for January or 01 for the first of the month), putting the field into edit mode will result in the PM being changed to AM. For example, 01/01/2022 12:00 PM with change to 01/01/2022 12:00 AM. 

If both the month and day are two digit numbers (e.g.: 10/10/2021), the issue does not occur.

This occurs in Microsoft Edge, Google Chrome and Mozilla Firefox (all latest builds as of 11/19/2021) running on Windows 10 as well as the Microsoft Lists PWA (which, in reality, is using Edge under the hood). 

It does not occur in any browser on Mac OS, so it's possible this is related to list sync (Project Nucleus). 

UPDATE: I believe this is directly related to Project Nucleus. I went into the settings for a few of the lists that were displaying this issue and disabled syncing (Advanced settings > Set "Offline Client Availability" to "No") and the issue promptly disappeared.

Here is a short video of the issue as its occurring: https://www.screencast.com/t/ELi3QPRMeIGD

[#2]

@Chad_V_Kealey 

Hi Chad did you ever get this issue sorted?

[#3]

Turning off syncing in a list eliminated the issue. However, at some point, the problem went away on its own. I've not noticed it or had it reported by users in the past year or so.

[#4]

Thank you very much for the solution. Indeed very strange behavior.

[#5]

@marrodgo I've seen an uptick in similar issues recently. Not exactly the same, but similar. For example, one particular user could access a list and create items, but one column (a lookup) wouldn't save the selected value, and these items never made it to the cloud. Only they could see them on their machine and others couldn't see them at all. Turning off syncing also "fixed" this. There are so many other variables it's hard to know the exact cause, but at least there's an easy way to avoid the problem. I just wish syncing were off by default. There are very few cases where I think it provides a benefit, but clearly it can cause issues.