When doing arithmetic operations with TIMESTAMP values, Firebird works with them as decimal number, where

• the integral fraction is the number of days
• the decimal fraction is the part of a day

For example:

• Two days: 2.0
• One hour: 1.0/24.0
• One minute: 1.0/1440.0
• One second: 1.0/86400.0

Extracting the smaller units from a TIMESTAMP value:

• Number of seconds: VALUE*86400.0
• Number of minutes: VALUE*1440.0
• Number of hours: VALUE*24.0

Don't forget the decimal point in the numbers (e.g. 1.0), otherwise the result will be integer. I spent a lot time debugging a calculation that didn't work because I was dividing by an integer. Since that time, I remember.