want to main start time and end time of the model pop up window

Hi friends,
   There been two user, that is u1 and u2. while u1 viewing model popup window (login using u1 login), at the same time u2 login to their accout and try to view popup , on that time it want to show "their are viewing for last 1 hour" like this i want to show... how can we achieve this.... How to capture the time of open and close popup window
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