Instead of reading the value directly, there is a GetValue() function that can read it from a different time. Documented in the old VFXPedia here:
https://www.steakunderwater.com/VFXPedi ... _.2F_frameInstead of adding a constant to time, you could add the results of another slider, allowing you to adjust the time offset easily.
…I see Sander ninja'd me. His solution is probably better in most cases, but I can see where your expression-linking method could also be useful, so if you want to stick with it, the above might be helpful.