Hi Jon

Thanks for your reply.

"these pre-defined timepoints are not actually based on time, right?"

Correct

"So if two people come in for Day Zero, they may not necessarily be on the same day or even the same month, right? So person #1 might have their Day Zero on January 29th, 2015 while person #2's Day Zero could have been a year ago. Is that more accurate to what you're trying to do?"

This is correct
In most cases we dont even know the date (or would take a lot of effort to dig up that date) we just have results for this subject at Day 0, Day1, Day 5 etc thats why for this particular study I want to avoid using dates "January 29th, 2015" but pre-defined timepoints like Day 0 etc.

Thanks for your help again, much appreciated.