[R] Help with POSIXct

[info at mathewanalytics.com]

I rarely work with dates in R, so I know very little about the
POSIXct and POSIXlt classes. I'm importing an excel file into R using
the RODBC package, and am having issues reformatting the dates.


1. The important info:
> sessionInfo()
R version 2.12.2 (2011-02-25)
Platform: i386-pc-mingw32/i386 (32-bit)

locale:
[1] LC_COLLATE=English_United States.1252  LC_CTYPE=English_United
States.1252   
[3] LC_MONETARY=English_United States.1252 LC_NUMERIC=C                   
     
[5] LC_TIME=English_United States.1252    

attached base packages:
[1] stats     graphics  grDevices utils     datasets  methods   base     

other attached packages:
[1] RODBC_1.3-2

loaded via a namespace (and not attached):
[1] tools_2.12.2


2. My question:

My data looks like the following once it is imported into R.

one <- odbcConnectExcel("marketshare.xls")
dat <- sqlFetch(one, "Sheet1")
close(one)

> dat
           F1 Marvel DC
1  2010-01-01     42 34
2  2010-02-01     45 34
3  2010-03-01     47 29
4  2010-04-01     45 32
5  2010-05-01     45 35
6  2010-06-01     42 34

Variable F1, is supposed to be Jan-10, Feb-10, Mar-10, etc.
However, in the process of importing the .xls file, R is reformating
the dates. How can I retrieve the original month-year format
that I have in my excel file.


3. While the R help list discourages against asking multiple questions in 
an inquiry, it'd be really helpful if someone could point me to any good
online
resources on the POSIXct and POSIXlt classes.