Try altering your subset operation from this:

with(DataSet, subset(DataSet, DataSet$NewDateTime < '2004-08-05 14:15:00'))

to this:

with(DataSet, subset(DataSet, DataSet$NewDateTime < as.POSIXct('2004-08-05
14:15:00')))

and see if you get the desired effect.

The statement DataSet$NewDateTime < '2004-08-05 14:15:00' is asking R to
find all of the rows in DataSet$NewDateTime that are less than the
*character* value '2004-08-05 14:15:00'. You need to convert that
*character* value to a POSIX time value first, using as.POSIXct(). Then you
can successfully carry out the comparison between the POSIXct values in
DataSet$NewDateTime and your newly created POSIX time value.

Because your character time value is listed in the standard POSIX format
(yyyy-mm-dd HH:MM:SS), you don't need to include the format information
(%y-%m-%d %H:%M:%S) in the as.POSIXct() function, which saves a little
typing. If it was in another format (mm-dd-yyyy) you'd need to use the
format argument in as.POSIXct() to make the character-to-POSIXct conversion
correctly.


On Thu, Sep 8, 2011 at 4:03 PM, Steve E. <searl@vt.edu> wrote:

> Dear R Community,
>
> I am new to R, and have a question that I suspect may be quite simple but
> is
> proving a formidable roadblock for me.  I have a large data set that
> includes water-quality measurements collected over many 24-hour periods.
> The date and time of sample collection are in a combined Date/Time field in
> the format yyyy-mm-dd hh:mm:ss.  I need to be able to subset the data for
> analysis of different date and time windows.  Thus far, I have tried
> casting
> the Date/Time field using several approaches, such as:
>
> DataSet$NewDateTime <- strptime(DataSet$DateTime, '%Y-%m-%d %H:%M:%S')
> DataSet$NewDateTime <- as.POSIXlt(strptime(DataSet$DateTime, '%Y-%m-%d
> %H:%M:S'))
>
> These instructions seem to cast the NewDateTime field correctly (at least
> it
> appears to be in the correct format, and I assume R sees the field as a
> date
> and a time) but I am then unable to subset the data using instructions such
> as:
>
> with(DataSet, subset(DataSet, DataSet$NewDateTime < '2004-08-05 14:15:00'))
> DataSubset <- subset(DataSet, DataSet$NewDateTime < '2004-08-05 14:00:00',
> select = DataSet)
>
> I have tried also separating the date and time fields in the input file,
> and
> casting with instructions such as:
>
> DataSet$NewTime <- strptime(DataSet$Time, '%H:%M:%S')
> DataSet$NewTime <- as.POSIXct(strptime(DataSet$Time, '%H:%M:%S'))
>
> but these seem to generate a NewTime field that contains today's date + the
> time data, and also will not subset based on date/time.
>
> I appreciate greatly any help and advice,
> Steve
>
> --
> View this message in context:
> http://r.789695.n4.nabble.com/help-subsetting-data-based-on-date-AND-time-tp3799933p3799933.html
> Sent from the R help mailing list archive at Nabble.com.
>
> ______________________________________________
> R-help@r-project.org mailing list
> https://stat.ethz.ch/mailman/listinfo/r-help
> PLEASE do read the posting guide
> http://www.R-project.org/posting-guide.html
> and provide commented, minimal, self-contained, reproducible code.
>



-- 
___________________________
Luke Miller
Postdoctoral Researcher
Marine Science Center
Northeastern University
Nahant, MA
(781) 581-7370 x318

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