GMT/UTC Considerations

I am attempting to create a spread sheet that will compensate for time zone
differences between the reference point (GMT) and teh user location. I have
come up with a way to do so by creating two user inpput fields for Ahead of
GMT and Behind GMT. My formula calculates the teh exact moment they enter the
desired information and even adds the day if it crosses midnight GMT. The
problem I am having is I use the NOW() funtion to grab the date/time of the
moment of entry. Each subsequent entry on any other field that uses the NOW
function recalculates any entry int the entire workbook that uses the NOW
function. Is there a way to prevent a cell that has already populated from
recalculating when another cell is entered?

Code example:

=IF(E6=0,"--",IF('5 minute'!E6>0,NOW()+TIME('5 minute'!E6,0,0),IF('5
minute'!E7>0,NOW()-TIME('5 minute'!E7,0,0))))

Where E6, E7, E8, etc. is the field that requires imput, '5 minute'!E6 is
the Hours Behind GMT static entry, and '5 minute'!E7 is the static input
Hours Ahead of GMT.