Understanding expression time

Time within an expression is always measured in seconds. The default time for any expression is the current composition time at which the expression is being evaluated. The following expressions both use the default composition time and return the same values:

thisComp.layer(1).position
thisComp.layer(1).position.valueAtTime(time)

To use a relative time, add a time value to the (time) argument. For example, to get a value at a time 5 seconds before the current time, use the following expression:

thisComp.layer(1).position.valueAtTime(time-5)