# Time Origin in Survival Analysis

I am new to survival analysis, and I am confused about time origin, or what's called calendar data format and survival data format.

I have the following dataset, subjects were continuously entering study since the study start, and we record the days after study start when they entered. I am trying to use the following two ways to plot survival curve, why they are so different? I am guessing it's about the time origin, but I am not understanding the story behind it.

Here is the dataset:

Here is the two ways of code:

Proc phreg data=ds; model actual_follow_up_days*censor(1)=;run;

Proc phreg data=ds; model Calendar_Days*censor(1)= \ entry=delay_entry ;run;

The difference here is between when the study started administratively (presumably Study Start) and when each Subject entered the study (presumably on Study Start plus the days of Delay_Entry). Then the Actual_Follow_Up_Days represents the follow-up time for each Subject after that Subject's entry into the study.
The Actual_Follow_Up_Days is almost certainly what you care about. Say you are studying how long Subjects survived after diagnosis of a type of cancer, with each Subject entering the study at the date of diagnosis. The fact that you happened to start your study on 8/5/2018 doesn't matter for any Subject's survival after diagnosis. If the diagnosis for an individual was on 8/5/2019 (Delay_Entry = 365) you would presumably care about survival after that date. I could imagine some study designs where survival from a common fixed date like Study_Start might be of interest, but without further information on your particular study design I think that would be unlikely.