Getting issue in finding duration between starting and ending trips

Hello R family, I have two columns 'started_at' and 'ended_at', of trips but when I fine the duration in mins between these two I find same duration for all trips. It should be different but I dont know why I am getting the same.

<- total_trips$trip_duration <- as.numeric(difftime(total_trips$ended_at,total_trips$started_at, units='mins'))
I am using this code.