I am not sure if i can answer the why question.
But would like to know, are you keeping the time zones in sync or the time itself in sync?
I believe the time itself needs to be in sync. I have not worked on setup where stores are across timezones, but would like to know that.
Please review note#1358353.1 - "SIM and ORPOS Integration: Should Server Times Be Synchronized?" for more information.
As part of the Open Web Application Security Project (OWASP) 'WebService' security guidelines, the SIM 'WebService' is protected against repeat attack by enabling the clock skew parameter of 'WebService' Security . Every incoming request is checked for freshness against the time on the server. If the timestamp of the message is greater than the configured clock skew, then the message will be rejected.
For this reason, it is imperative that both the SIM and POS server times are synchronized.
You can try the following two options to adjust the clock skew to what is required on the sim server .
First is setting clock skew for the webservices. Following are the steps to do this.
Open the webLogic admin console
On the left pane click on the domain name
From the right pane click Web Service Security
Click on the web service security configuration
Select the timestamp tab
Modify the clock skew parameter
If the above does not work, you can try the following option as well.
Go to Security Realms
Then edit 'DigestExpirationTimePeriod' value according to your requirement.
Restart the server .
I hope this helps.