Time within an expression is always in composition time (not layer time) and is 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 an incremental time value to the time argument. For example, to get the Position value 5 seconds before the current time, use the following expression:

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

Default time references to properties in nested compositions use the original default composition time, not remapped time. However, if you use the source function to retrieve a property, the remapped time is used.

For example, if the source of a layer in the parent composition is a nested composition, and in the parent composition you have remapped time, when you access the position values of a layer in the nested composition with the following expression, the position values use the composition’s default time:

  comp("nested composition").layer(1).position

However, if you access layer 1 using the source function, the position values use the remapped time:

  thisComp.layer("nested composition").source.layer(1).position