I have two timestamps in character form that I want to convert to a POSIX format in R.
1: "2013-03-30 17:45:00"
2: "2013-03-31 02:05:00"
It looks like a DST issue. I assume from your name you are from Sweden or thereabouts. There was no time in Sweden between 2am and 3am on the 31st March 2013, as the clocks went forward then.
as.POSIXct("2013-03-31 02:05:00",format="%Y-%m-%d %H:%M:%S", tz="Europe/Stockholm")  NA
This is true for anyone on Central European Time (CET).