I have cells with dates in this format:
DD/MM/YYYY 00:00:00 and I want to add 3 hours because I'm in a different GMT zone.
Is possible to use dates in a formula?
Something like: 30/12/2012 22:15:00 should become: 31/12/2012 01:15:00
All date/time values in spreadsheets are internally handled as floating point values. To add 3 hours to a date/time just add (3/24) to the original date/time.
This also works in MS Excel.
See also: Convert a date in Google Spreadsheet?
Adds 3 'Hours', 0 'Minutes', 0 'Seconds' to the date/time value of A1
Additionally if only you use the main sheet you can go into the sheet settings and change it's timezone accordingly.
I do this for my work schedule to track different projects:
Three columns A B C where column A is text, B is hours, C is minutes.
In Time 8:00 AM is B2
Out Time 3:45 PM