The simplest way to truncate the file is with truncate or just ">file", since writes are small enough to be atomic.  The sa utility, which also goes back to v7, will summarize process accounting data and write it to a different file; it can then report on either unsummarized data or summarized data before unsummarized data).  Unfortunately Linux has broken the wtmp/utmp convention of "no logfile, no logging", so a cron job to truncate wtmp is your only man.

John Cowan          http://vrici.lojban.org/~cowan        cowan@ccil.org
Original line from The Warrior's Apprentice by Lois McMaster Bujold:
"Only on Barrayar would pulling a loaded needler start a stampede toward one."
English-to-Russian-to-English mangling thereof: "Only on Barrayar you risk to
lose support instead of finding it when you threat with the charged weapon."