Hello Anand,
Changing the timezone appearing in the GUI is supported and always has been. This is accomplished in the User Preferences > Time Zone Preferences in the dropdown under your username in the Management Center. CSCuu70250 fixes a bug where the GUI timezone couldn't couldn't be set on a Firepower module managed by ASDM. The code which you are running is affected with the bug . Thus please upgrade the device to 6.0.1 to accomplish the capability.
http://www.cisco.com/c/en/us/td/docs/security/firepower/601/relnotes/firepower-system-release-notes-version-601.html
Just FYI ,Changing the timezone on the Linux backend is not supported and should never be done. Changing the timezone using a Linux command will cause unexpected results and will require you to rebuild the Firepower module to correct the problem.
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Regards
Jetsy