following this guide I've disabled ntpd, now I wonder if is possible to sincronize time at MB startup (instead having a daemon constantly running)
Your mybook should automatically sync at startup anyway, I also set up a cron job to sync it once a day since I always leave it on.
I´ve done it like you,
my solution for time sync every day is here.
MBWE II (blue rings), 2x 1TB WD10EARS in Raid 1 mode
Hacks: SSH, optware, spindown
Applications: dokuwiki, nightly backup script