If you take a set of POSIXct-format time stamps in R and convert them to text strings (via as.character() ), the midnight time stamps will be shown with just the date and no 00:00 time value. If you subsequently try to convert a vector of text time stamps back to POSIXct, R may decide that your midnights aren’t midnights because they lack a 00:00 value, so you might want to add it back.

This little function should accomplish that:

 # Function to add 00:00 to text representations of midnight timestamps. Input times should be text strings (characters)
fixMidnight = function(times){
	ifelse(grepl("^\\d{4}-\\d{2}-\\d{2}$", tstamps), 
	paste0(tstamps," 00:00"),
	tstamps)
}

You supply it a vector of character time values (like “2024-09-10 23:00” and “2024-09-11”), and this will return character values where the midnight time value will now read “2024-09-11 00:00”, while the times that already had hours:minutes will remain unchanged: “2024-09-10 23:00” will return exactly the same as it was supplied to the function.

As an explanation, the grepl pattern:

   "^\\d{4}-\\d{2}-\\d{2}$"

is looking for the typical POSIX representation of a year-month-day, i.e. YYYY-MM-DD, and only returning entries that end after the 2-digit month, instead of those that continue with hour:minute values.