Simple technique to correct the date/time of a Big-IP. F5 will tell you to take your unit into single user mode, this is not neccessary. I have discovered that you can update the time of an standby unit in multiuser mode.
You can update the clock in multiuser mode with ntpdate.
Syntax: ntpdate -b (time server IP/FQDN)
Manual page for ntpdate: http://www.eecis.udel.edu/~mills/ntp/html/ntpdate.html