Make sure the selected Time Zone on the Date/Time Group page has an asterisk on its name. This means it will work with newer phone loads and also older phone loads. This may be the issue.

You mentioned that you set up a date/time group. If you consider the option of configuring a "Phone NTP reference"

Here is a procedure that you can try to create a test one, and assign    it to a test ip phone to see if that works fine in your environment:

step 1: ccm admin > system > phone NTP reference > add new > enter the ip address of your NTP server (you may enter the ip address of the publisher) >select the "directed broadcast" option > save.

step 2:  ccm admin > system > Date/Time group > add new > create a new Date/Time group and please use the "phone NTP reference" that you just created >save.

step 3: ccm admin > system > Device Pool > add new > create a new device pool (please specify there the "Date/Time Group" that you just created).

step 4: Assign the new device pool to an ip phone that can be used for testing.