Extracting time from POSIXct

Question

How would I extract the time from a series of POSIXct objects discarding the date part   For instance  I have   times  lt - structure c 1331086009 50098  1331091427 42461  1331252565 99979   1331252675 81601  1331262597 72474  1331262641 11786  1331269557 4059   1331278779 26727  1331448476 96126  1331452596 13806   class   c  POSIXct     POSIXt      which corresponds to these dates    2012-03-07 03 06 49 CET   2012-03-07 04 37 07 CET    2012-03-09 01 22 45 CET   2012-03-09 01 24 35 CET    2012-03-09 04 09 57 CET   2012-03-09 04 10 41 CET   2012-03-09 06 05 57 CET   2012-03-09 08 39 39 CET   2012-03-11 07 47 56 CET   2012-03-11 08 56 36 CET    Now  I have some values for a parameter measured at those times  val  lt - c 1 25343125e-05  0 00022890575            3 9269125e-05  0 0002285681875            4 26353125e-05  5 982625e-05            2 09575e-05  0 0001516951251            2 653125e-05  0 0001021391875    I would like to plot val vs time of the day  irrespectively of the specific day when val was measured   Is there a specific function that would allow me to do that

User · Answer

I can t find anything that deals with clock times exactly  so I d just use some functions from package lubridate and work with seconds-since-midnight   require lubridate  clockS   function t  hour t  3600 minute t  60 second t   plot clockS times  val    You might then want to look at some of the axis code to figure out how to label axes nicely

User · Answer

There have been previous answers that showed the trick  In essence    you must retain POSIXct types to take advantage of all the existing plotting functions if you want to  overlay  several days worth on a single plot  highlighting the intra-daily variation  the best trick is too     impose the same day  and month and even year if need be  which is not the case here    which you can do by overriding the day-of-month and month components when in POSIXlt representation  or just by offsetting the  delta  relative to 0 00 00 between the different days   So with times and val as helpfully provided by you      impose month and day based on first obs ntimes  lt - as POSIXlt times       convert to  POSIX list type  ntimes mday  lt - ntimes 1  mday    and  mon if it differs too ntimes  lt - as POSIXct ntimes      convert back  par mfrow c 2 1   plot times val      old times plot ntimes val     new times   yields this contrasting the original and modified time scales

User · Answer

You can use strftime to convert datetimes to any character format    gt  t  lt - strftime times  format   H  M  S    gt  t   1   02 06 49   03 37 07   00 22 45   00 24 35   03 09 57   03 10 41    7   05 05 57   07 39 39   06 47 56   07 56 36    But that doesn t help very much  since you want to plot your data   One workaround is to strip the date element from your times  and then to add an identical date to all of your times    gt  xx  lt - as POSIXct t  format   H  M  S    gt  xx   1   2012-03-23 02 06 49 GMT   2012-03-23 03 37 07 GMT    3   2012-03-23 00 22 45 GMT   2012-03-23 00 24 35 GMT    5   2012-03-23 03 09 57 GMT   2012-03-23 03 10 41 GMT    7   2012-03-23 05 05 57 GMT   2012-03-23 07 39 39 GMT    9   2012-03-23 06 47 56 GMT   2012-03-23 07 56 36 GMT    Now you can use these datetime objects in your plot   plot xx  rnorm length xx    xlab  Time   ylab  Random value         For more help  see  DateTimeClasses

User · Answer

The data table package has a function  as ITime   which can do this efficiently use below   library data table  x  lt -  2012-03-07 03 06 49 CET  as IDate x    Output is  2012-03-07  as ITime x    Output is  03 06 49

User · Answer

The time t value for midnight GMT is always divisible by 86400  24   3600    The value for seconds-since-midnight GMT is thus time    86400   The hour in GMT is  time    86400    3600 and this can be used as the x-axis of the plot   plot  as numeric times     86400  3600  val      To adjust for a time zone  adjust the time before taking the modulus  by adding the number of seconds that your time zone is ahead of GMT   For example  US central daylight saving time  CDT  is 5 hours behind GMT   To plot against the time in CDT  the following expression is used   plot   as numeric times  - 5 3600     86400  3600  val

User · Answer

Many solutions have been provided  but I have not seen this one  which uses package chron   hours   times strftime times  format   T    plot val hours     sorry  I am not entitled to post an image  you ll have to plot it yourself

User · Answer

Here s an update for those looking for a tidyverse method to extract hh mm  ss sssss from a POSIXct object  Note that time zone is not included in the output  library hms  as hms times

[r] Extracting time from POSIXct

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