I don't think you need to. I just opkg install ntp and ntpdate, then try to
run ntpdate (like this: 'ntpdate pool.ntp.org' early on in the boot process.
If it fails I just try again every minute or two until it succeeds. ntp will
then slew the time based on measured frequency or something (i.e. make
continual *rate* adjustments but no big discontinuous *setting* adjustments).
So your time will be wrong until an ntpdate command succeeds. But once it does
it should remain fairly correct for a while, even if the network goes away.