why are you reconfiguring the timezone every time? Once should be enough
ntpdate-debian will set the time once. I have no idea how good the internal clock is, but for a better timekeeping you might want to run ntpd. It will continuously adjust the time. It also keeps a drift file, that has the average drift of the internal clock. If the internal clock isn't flactuating too much, this should (after a while) provide a good enough correction even without network.