Rounding time to nearest quarter hour

Question

I have a vector of POSIXct values and I would like to round them to the nearest quarter hour. I don\'t care about the day. How do I convert the values to hours and minutes?<

Answer 1

You can use round. The trick is to divide by 900 seconds (15 minutes * 60 seconds) before rounding and multiply by 900 afterwards:

a <-as.POSIXlt("2012-05-30 20:41:21 UTC")
b <-as.POSIXlt(round(as.double(a)/(15*60))*(15*60),origin=(as.POSIXlt('1970-01-01')))
b
[1] "2012-05-30 20:45:00 EDT"

To get only hour and minute, just use format

format(b,"%H:%M")
[1] "20:45"

as.character(format(b,"%H:%M"))
[1] "20:45"