How to subtract 7 hours from a DATE datatype column displaying Date & Time

Greetings,

I have a huge database in MS Excel and one of the fields captioned "Start Date" has Date & Time in this "DATE" format:
3/14/10 1:30 PM

My problem is that the data I have is in GMT -7 timezone. I need to redo this data on GMT. This means, I need to subtract 7 hours from the 'START DATE' column and I expect that the dates would automatically be adjusted by MS Excel, where required after taking -7 hours difference into account.

Could anyone help how to achieve this? When I tried doing A1-7 (Assuming date is written in A1 cell), the result is subtraction of 7 days in the date. i.e. if A1 was 3/14/10 1:30 PM, it changes to 3/07/10 1:30 PM while I want it to be 3/14/10 6:30 AM.

How can I do this? Please advise.

Thanks,
Syed