weird negative seconds on checkpoint message.

Hi!
I have the next messege:
LOG: checkpoints are occurring too frequently (-21093 seconds apart)

Postgres 8.3.11 is running on SCO Openserver 5.0.7

and this is my checkpoint configuration:
# - Checkpoints -

checkpoint_segments = 32 # in logfile segments, min 1, 16MB each
checkpoint_timeout = 15min # range 30s-1h
checkpoint_completion_target = 0.7 # checkpoint target duration, 0.0
- 1.0
#checkpoint_warning = 30s # 0 is off

Any idea, why i get negative value????
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