unable to logon to my software system when my system time zone is in PST(server is in EST)

Hi all,

I am unable to logon to my software system(health care system) when my system time zone is in PST(Im in EST time zone ORLANDO ).
 Here my server time zone is in EST(ORLANDO).
When i changed(reduced) my system(reduced PST zone by 3hrs) time by 3hrs  i am able to logon to my system.

when i changed my time zone to EST the system works perfectly.

Can any one tell me  why i am not able to login.

thanks in advance
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