determine mon_timeofday (of Monday midnight)

# using stat_file...)
seek to end

while (line_timeofday > mon_timeofday)
  seek back some amount
  read a line

while (line_timeofday < mon_timeofday)
  seek forward some smaller amount
  read a line

# now we're at either: line_timeofday == mon_timeofday, or
#                      line_timeofday > mon_timeofday (just a bit...)
