[Haskell-beginners] Help with Data.Time

[Hong Yang]

I want to calculate the length of time between time ta and time tb.
Time ta is a String looking like "N:NNAM" or "N:NNPM."
I need tb to be 9:30AM on the current date.

I think the flow should be
1) parse ta to utcTimeA :: UTCTime
2) get current date and form tb to utcTimeB :: UTCTime
3) call "diffUTCTime utcTimeA utcTimeB"

So my program looks like as follows in a "do" expression:

    let utcTimeA = fromJust (parseTime defaultTimeLocale "%R%P" ta :: Maybe
UTCTime)
    currentTime <- getCurrentTime
    let day = utctDay currentTime
    let utcTimeB = UTCTime day "09:30:00.0000000 UTC"
    let timeElapsed = diffUTCTime utcTimeA utcTimeB

There are two difficulties I could not get around:
1) It seems that parseTime returns "Nothing". Why?
2) I knew "09:30:00.0000000 UTC" in the statement of "let utcTimeB ..." is
incorrect, because it is a String instead of expected DiffTime type. After
consulting Data.Time module (it just say "Data DiffTime"), I still cannot
figure out how to get utcTimeB.

Can someone help me?

Thanks,

Hong
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