jquery countdown changes when user change its system time

i'm using jquery countdown with php. i have given an end date which is going to the countdown. my problem is lets suppose 1 hour left is showing in countdown but when a user change its system time the countdown changes. like if a user back his time 1 hour then the counter will display the 2 hours left. is there any way to get the server time for more accurate time not the user system time. please help.

how can i get server time not user system time?