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 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;