There are several things you need to verify here.
1. Is NTP set correctly on UC and syncronized?
2. Do you have the correct time zone set on the server?
3. Do you have the correct time zone set on the user (if they aren't in the same timezone as the server)?
I know there are a few one off timezone settings that don't follow DST, and as a result, a user erroneously pointed at one of these might have time off by an hour about half the year.