Calender time varies for Users

Hello, I am new to SugerCRM and having a problem I can not fix. I installed SugerCRM in my server and gave access to different laptops users of my office. When I create a meeting on the calender and invite my colleagues, two of my colleagues get a 4 hours early invitation on their calender.
This might be a silly problem but becoming a big issue for me. Please help.

Are the user time zones all set up correctly?. You set this in the user preferences.