When visiting websites browser says can't validate security certificate.
Check the computer's time and see it is on the correct date and time.
To set the time on the Mac open the Apple Menu on the top left corner of the screen and select System Preferences.
Go to the icon of a clock labeled 'Date & Time'. On that screen the check box labeled 'Set date and time automatically' should be checked. It may be greyed out. To change this unlock the pad lock in the corner by clicking and typing in the computer's administrator credentials. Uncheck the 'Set date and time automatically', then recheck the box. The computer should have the correct time. If not manually enter the time.