GetSecondsPassed

Link to TES4 Construction Set Wiki: GetSecondsPassed.

In Fallout 3, GameDaysPassed is a float with enough precision to effectively measure the passage of seconds over short periods of time. Under those circumstances, this can be used as an alternative to GetSecondsPassed. Consider:

float MyTimer

begin GameMode
    if GameDaysPassed < MyTimer
        return
    else
       ; Do stuff.

       ; Reset the timer to 5 seconds (24 hours of 60 minutes of 60 seconds in a day).
        set MyTimer to GameDaysPassed + (1.0 / 24.0 / 60.0 / 60.0) * 5.0
    endif
end

This is the same as this:

float MyTimer

begin GameMode
   ; Keep track of time.
    set MyTimer to MyTimer - GetSecondsPassed

    if MyTimer > 0
        return
    else
       ; Do stuff.

       ; Reset the timer to 5 seconds.
        set MyTimer to 5
    endif
end

It is also slightly more efficient to use GameDaysPassed because you do not need to update a variable every frame, you only have to do the check - saving the script an operation. A minor note, but nonetheless notable.

However, for cases where the timer needs to be accurate to fractions of a second, GetSecondsPassed is the best choice, as the GameDaysPassed is not precise enough for this purpose.