Yitz, thanks for the detailed answer!
May be I should have formulated my question differently. All I actually need is some way to get *seconds* since epoch from the  system, so I could manipulate them as integers.
Correct me if I am wrong, but UTCTime does not help here.
The only way I found to get seconds form epoch using existing today GHC libraries is that:

utcStr = "Wed, 07 Dec 2011 10:10:05 +0000"
posixSecondsIntg = read posixSecondsStr :: Integer
posixSecondsStr =  init $ show (cvtUTCtoSec  utcStr) -- throw away 's' from posix seconds string, s\
uch as "1235657s"

cvtUTCtoSec utcTimeStr = utcTimeToPOSIXSeconds utcTime where
        utcTime = fromJust $ tryParseTime utcTimeStr

timeFormat1 = "%a, %d %b %Y %T %z"
timeFormat2 = "%m/%e/%Y %l:%M:%S %p"

tryParseTime :: String -> Maybe UTCTime
tryParseTime timeStr = tryFormat (parseTime defaultTimeLocale timeFormat1 timeStr :: Maybe UTCTime)
   where
     tryFormat time
        | time == Nothing = parseTime defaultTimeLocale timeFormat2 timeStr :: Maybe UTCTime
        | otherwise = time


Not a very easy way, isn't it?

On Sun, Dec 11, 2011 at 9:22 PM, Yitzchak Gale <gale@sefer.org> wrote:
dokondr wrote:
> When I try to read POSIXTime...
> No instance for (Read POSIXTime)...
> What should I do to provide Read instance for POSIXTime?

Short answer: if you are thinking about this as a moment
in time that could be parsed from the usual kind of
string representation for that, you probably want to
use UTCTime in your data type, not POSIXTime.

If you really, really want to represent it internally
as POSIXTime, then you should read those strings
as UTCTime and then convert them to POSIXTime
to store in your data type. I.e., in that case don't
make your data type an instance of Read.

POSIXTime is just a type alias for NominalDiffTime,
i.e., a quantity of time between two moments.

This is what the Show instance looks like:

Prelude Data.Time> realToFrac 1000000 :: NominalDiffTime
1000000s

By convention, the Read instance would expect a
string in that format. Generally people aren't interested
in that, so there is no Read instance.
Even if you did want to parse that, you would just
parse it as a number and then use realToFrac, as I
did above.

The rule of thumb is: always represent moments in
time as a UTCTime.

Regards,
Yitz