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Say if I have an array that contains DateTimes:

> head(DateTime)
[1] "2010-04-23 13:00:00 UTC" "2010-04-23 14:00:00 UTC" "2010-04-23 15:00:00 UTC"
[4] "2010-04-23 16:00:00 UTC" "2010-04-23 17:00:00 UTC" "2010-04-23 18:00:00 UTC"

and another array of hourly values:

> head(hour_vals)
[1] 20 20 20 20 20 20

How would I change the hours in DateTime to be equivalent to the hours in hour_vals, for example:

> head(FinalResult)
[1] "2010-04-23 20:00:00 UTC" "2010-04-23 20:00:00 UTC" "2010-04-23 20:00:00 UTC"
[4] "2010-04-23 20:00:00 UTC" "2010-04-23 20:00:00 UTC" "2010-04-23 20:00:00 UTC"
user2355358
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1 Answers1

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This is one way:

DateTime.lt <- as.POSIXlt(DateTime)
DateTime.lt$hour <- 20
DateTime <- as.POSIXct(DateTime.lt)

Unlike POSIXct, which stores datetimes as a single number (seconds since the epoch), POSIXlt stores them as tuples of seconds, minutes, hours, etc., which can be accessed directly (see ?POSIXlt for details). POSIXct has a smaller memory footprint, and is therefore the preferred datatype for datetime objects, but for tasks such as this, temporarily converting to POSIXlt is simple and effective.

Matthew Plourde
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    A word of caution: altering `POSIXlt` objects like this can potentially yield nonsensical date-times: e.g. `DateTime.lt <- as.POSIXlt("2013-03-10 01:00:00", tz="America/Chicago"); DateTime.lt$hour <- 2`. DST started at 2am in this TZ, so the updated `DateTime.lt` didn't actually exist. – Joshua Ulrich May 30 '13 at 14:22