[R] POSIXct and chron issues with tz

[Sherri Heck]

 Dear All-

I am trying to merge two data files - they have different date formats 
and different times zones.  I need to match up the date/time of the 
datasets and then invoke a conditional statement, such as: if dataC$mph 
is >= 12 then keep dataM$co23 for the corresponding time/date stamp.

snippets of data files:

*dataC.txt*
 LST   in     mph    Deg   DegF  DegF2    %    volts   Deg    mph2   w/m2
0506010000   0.00   13.6  218.1   36.8   -999   65.1   -999   -999   
18.2    0.2
0506010005   0.00   12.9  214.3   36.8   -999   65.5   -999   -999   
16.9    0.2
0506010010   0.00   14.4  215.7   36.9   -999   65.4   -999   -999   
20.4    0.2

*dataM.txt*
doy yr mon day hr hgt1 hgt2 hgt3 co21 co22 co23 sig1 sig2 sig3 dif flag

244.02083 2005 09 01 00 2.5 5.8 9.1 -999.99 -999.99 -999.99 -999.99 
-999.99 -999.99 -999.99 PRE
244.0625 2005 09 01 01 2.5 5.8 9.1 -999.99 -999.99 -999.99 -999.99 
-999.99 -999.99 -999.99 PRE
244.10417 2005 09 01 02 2.5 5.8 9.1 -999.99 -999.99 -999.99 -999.99 
-999.99 -999.99 -999.99 PRE

*Code:*

z <- read.zoo("dataC.txt", header = TRUE, na.strings = -999,      
        format = "%y%m%d%H%M", FUN = 
as.chron,                                                                
        colClasses = c("character", rep("numeric", 10)))

DF <- read.table("C:/R/dataM.txt", header = TRUE, na.strings = -999.99)
tt <- with(DF, chron(paste(mon, day, yr, sep = "/"),
       paste(hr, 0, 0, sep = ":")))
       timeconv <-  as.POSIXct(tt, tz="")
       z2 <- zoo(data.matrix(DF), timeconv)


*example of output of data files*

 >z[1,]
                    in.  mph   Deg DegF DegF2   X. volts Deg.1 mph2 w.m2
(06/01/05 00:00:00)   0 13.6 218.1 36.8    NA 65.1    NA    NA 18.2  0.2


 > z2[1,]
                         doy   yr mon day hr hgt1 hgt2 hgt3 co21 co22 
co23 sig1 sig2 sig3 dif flag
2005-08-31 18:00:00 244.0208 2005   9   1  0  2.5  5.8  9.1   NA   NA   
NA   NA   NA   NA  NA    1

*#merging data files z, z2*
m <- merge(z, z2, all = c(FALSE, TRUE))
m[m==-999.99]<-NA
m2 <- m[m$mon %in% c(6,7,8),]

*returns the following warning message:*
Warning message:
In merge.zoo(z, z2, all = c(FALSE, TRUE)) :
  Index vectors are of different classes: chron POSIXt

I cannot find a way to deal with the timezone in chron.  But, I found 
through the help pages that POSIXct deals with tz.  However, it doesn't 
seem these two different formats are compatible to merge.  Any thoughts 
would be appreciated.

cheers,

sherri