Look in belchertown.py near line 3330 for those. To me, they're just noise messages you can quiet down by just commenting those two lines out. You might need to restart weewx after doing so, as Belchertown seems to require that more than some other skins.
if observation == "haysChart":
start_ts = int(start_ts)
end_ts = int(end_ts)
# Set aggregate interval based on timespan and make sure it is
# between 5 minutes and 1 day
logging.debug("Start time is %s and end time is %s" % (start_ts, end_ts))
aggregate_interval = (end_ts - start_ts) / 360
if aggregate_interval < 300:
aggregate_interval = 300
elif aggregate_interval > 86400:
aggregate_interval = 86400
logging.debug("Interval is: %s" % aggregate_interval)