[R] Dates as POSIXt

[Prof Brian Ripley]

You cannot have a POSIXlt column in a data frame: if you did your 
homework you would know it is because it is length-9 list.

I don't know why str() is reporting POSIXt as a class, and your example 
is not reproducible.  Normally date-times have class

 > class(Sys.time())
[1] "POSIXct" "POSIXt"

and I suspect you actually have a POSIXct column.  But the order of 
these two classes is not important and has changed over the years.

On 05/11/2012 07:51, Daniel Haugstvedt wrote:
> When I try to do linear interpolation between financial contracts with maturities on different dates in different months I have come across some behavior I haven't seen before.
>
> I have a data frame in R which is loaded from an access database so I can't provide a working example. It was loaded using this code:
>
>> dbPath <- "H:/pathToDB/DB.mdb"
>> channel <- odbcConnectAccess(dbPath)
>> DF  = sqlFetch(channel,'nameOfTable')
>
> When I look at the Date column I get this result
>> str(DF$Date)
> POSIXt[1:25311], format: "2003-09-03 06:00:00" "2003-09-03 06:00:00" ...
>
> I have newer seen data as POSIXt, only as POSIXct or POSIXlt. It is the behavior of this class is that I would like more information about. Online searching have only told me that it is a virtual class.
>
> When I do some calculations to get the dates of maturity into the data frame I find this behavior. (For simplicity assume that the only month is March.)
>
>> DF[,"DateOfMaturity"] = NA
>> DF[,"DateOfMaturityPrevious"] = NA
>> DF[,"DateOfMaturityNext"] = NA
>
>> maturityFeb = 14
>> maturityMar = 16
>> maturityApr  = 15
>
>> yearTmp = as.POSIXlt(DF$Date)$year+1900
>> DF$DateOfMaturity = as.POSIXct(strptime(paste(yearTmp,03,maturityMar ), "%Y %m %d"))
>> DF$DateOfMaturityPrevious =  as.POSIXct(strptime(paste(yearTmp,02,maturityFeb ), "%Y %m %d")
>> DF$DateOfMaturityNext = as.POSIXct(strptime(paste(yearTmp,04,maturityApr ), "%Y %m %d"))
>
> which works fine and gives me the dates I want but it is not readable with human eyes. When I try
>
>> DF$DateOfMaturity =  as.POSIXlt(strptime(paste(yearTmp,03,maturityMar ), "%Y %m %d"))
>> DF$DateOfMaturityPrevious =  as.POSIXlt(strptime(paste(yearTmp,02,maturityFeb ), "%Y %m %d")
>> DF$DateOfMaturityNext =  as.POSIXlt(strptime(paste(yearTmp,04,maturityApr ), "%Y %m %d"))
>
> it breaks my DF
>
>> str(DF$DateOfMaturity)
> List of 2015
> $ : num [1:2015] 0 0 0 0 0 0 0 0 0 0 ...
> $ : int [1:2015] 0 0 0 0 0 0 0 0 0 0 ...
> $ : int [1:2015] 0 0 0 0 0 0 0 0 0 0 ...
> $ : int [1:2015] 16 16 16 16 16 16 16 16 16 16 ...
> $ : int [1:2015] 0 0 0 0 0 0 0 0 0 0 ...
> $ : int [1:2015] 104 104 104 104 104 104 104 104 104 104 ...
> $ : int [1:2015] 5 5 5 5 5 5 5 5 5 5 ...
> $ : int [1:2015] 15 15 15 15 15 15 15 15 15 15
>   .
>   .
>   .
> [list output truncated]
>
> Now I wonder why I can't use POSIXlt in my data frame (I know I shouldn't but that is not the question) and if I can use POSIXt like the original data?  It is human readable but also suited for calculation (e.g. DF$Date >  as.POSIXct("2005-12-01") works nicely.
>
>
> Best regards
> Daniel Haugstvedt
> Ph.D. student
> NTNU, Trondheim, Norway
>
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> and provide commented, minimal, self-contained, reproducible code.
>


-- 
Brian D. Ripley,                  ripley at stats.ox.ac.uk
Professor of Applied Statistics,  http://www.stats.ox.ac.uk/~ripley/
University of Oxford,             Tel:  +44 1865 272861 (self)
1 South Parks Road,                     +44 1865 272866 (PA)
Oxford OX1 3TG, UK                Fax:  +44 1865 272595