I am using the package "survival" to fit a cox model with time intervals (intervals are 30 days long). I am reading the data in from an xlsx worksheet. I keep getting the error that says my stop time must be greater than my start time. The start values are all smaller than the stop values.

I checked to make sure these are being read in as numbers which they are. I also changed them to integers which did not solve the problem. I used this code to see if any observations met this criterion:

a <- a1[which(a1$end_time > a1$start_time),]

About half the dataset meets this criterion, but when I look at the data all the start times appear to be less than the end times.

Does anyone know why this is happening and how I can fix it? I am an R newbie so perhaps there is something obvious I don't know about?

model1<- survfit(Surv(start_time, end_time, censor) ~ exp, data=a1, weights = weight)

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