How to average time series data around regular intervals

x <- read.table(text = "2012-07-22 12:12:00, 21

# get minimum at an hour granularity
tMin <- trunc(min(x$V1), units = 'hour')
# back off 7.5 minute
tMin <- tMin - (7.5 * 60)
# create sequence for 'cut'
cSeq <- seq(tMin, max(x$V1) + (7.5 * 60), by = '15 min')
# now split and average
cCut <- cut(x$V1, cSeq)
# compute means
tapply(x$V2, cCut, mean)

Hi,

I'm pretty new to R and have run into a task which although I'm certain is
within R's capabilities, falls outside of mine. :-) Consider the following
data set:

2012-07-22 12:12:00, 21
2012-07-22 12:15:00, 22
2012-07-22 12:18:00, 24
2012-07-22 12:39:00, 21
2012-07-22 12:45:00, 25
2012-07-22 12:49:00, 26
2012-07-22 12:53:00, 20
2012-07-22 13:00:00, 18
2012-07-22 13:06:00, 22

My task involves creating a data set which *averages* these values at a
resolution of 15 minutes, meaning that I need to average the values falling
within 7.5 minutes of a 15 minute increment. Therefore given the above data
set I need to treat it as three groups:

2012-07-22 12:12:00, 21
2012-07-22 12:15:00, 22
2012-07-22 12:18:00, 24

2012-07-22 12:39:00, 21
2012-07-22 12:45:00, 25
2012-07-22 12:49:00, 26

2012-07-22 12:53:00, 20
2012-07-22 13:00:00, 18
2012-07-22 13:06:00, 22

The end result should look like this:

2012-07-22 12:15:00, 22.33
2012-07-22 12:30:00, NA <- Because this 15 minute slot did not previously
exist
2012-07-22 12:45:00, 24
2012-07-22 1:00:00, 20

Any help much appreciated. I've been working on this for several hours with
little success. I'm able to identify the missing (NA) value using zoo/xts
but can't seem to sort out the averaging matter.

Thanks so much!
Jason

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