ReadTime is a hack which shows you the Total Power On Time (TPOT) of your Mac, i.e. the total time your mac has been on EVER. Your Mac updates the TPOT every five minutes.
Older types of Macs (Plus, SE, II...) don't keep track of the power on time and the Total Power On Time for these macs is zero.
The TPOT is stored in the Parameter RAM (PRAM) of your Mac. If for some reason this PRAM was ever reset (battery died, motherboard malfunction, some badly written pramzap utilities), the TPOT may be not be correct and be lower that you'd expect.
If Readtime indicates a very low TPOT (a few hours or so), this could indicate a problem with your PRAM or PRAM battery.
