/var/log/wtmp always reseting to 0

[questions]

Howdy,
	I'm running FreeBSD 4.10 my /var/log/wtmp file gets reset to 0 Kb
(as well as the wtmp.X.gz files) at the end/beginning of each month. I
noticed this last month when the monthly run output notice sent to roots
mail stated =
__________________Snip_____________________
TO:root at davez.org
Subject: davez.org monthly run output

Doing login accounting:
        total                0.00

-- End of monthly output --
_______________End Snip___________________

At first I thought maybe a isolated case but this month the same thing
happened, the end of month accounting shows no logins and when I check the
log files they are all empty 0Kb prior to todays activity.

Anyone else seen this or have any ideas what may be wrong?

Thanks in advance for any help provided -

Dave