How to convert Year-Month-Day into continuous values

Question

I am trying to do an analysis on weather events from 2016 to 2021. So I want to convert this time period from 2016 to 2021 to days such that StartTime[1] = 2016-01-06 23:14:00 is day 1, all the way to StartTime[length(df$StartTime)] = 2021-12-28 12:18:00 is day 1822 (5*365 - 3).

I've converted the StartTime and EndTimes to POSIXct types and I tried to write a function to iterate over the dataframe to run difftime(StartTime[i], StartTime[1], units="days")

The time data comes in this format StartTime[1] = 2016-01-06 23:14:00

whereas StartTime[length(df$StartTime)] = 2021-12-28 12:18:00

This is the function I tried to write to do this..

cont_days<-function(date) {
  for (i in df)
  days<-difftime(date[i], date[1], units="days")
  return(days)
}

df$continous_days<-cont_days(df$StartTime.UTC.)

but I'm getting this error message that I've been scratching my head over.

Error in `$<-.data.frame`(`*tmp*`, continous_days, value = c(NA_real_, : 
replacement has 2453 rows, data has 2455

Edit: Okay so I did df<-na.omit(df) to ensure I removed all the NA rows.