timezone between NY and London
I think the best way would be to store the last login time as an UTC-value in the DB and use PHP to convert that value to the visitor's timezone. I'd always store any timestamp data as UTC in the DB, also to avoid problems with DST and all that stuff. I think in terms of speed there is not much difference if the DB calculates the display value for you or if your PHP script does it. However since your PHP script is your presentation layer, it should take care of date formatting instead of the DB.