Why does clock reset itself each time I start the vehicle?

Sometimes if the battery is weak enough that could cause a drain and suddenly the memory fails and then it resets to 12 oclock.Also the stereo system uses two 12 volt positive feeds one is a constant that many times is fused by the same fuse that feeds the lighter socket but also many times only one of the fuses on the fuse chart will mention the stereo when in fact another fuse is feeding a different circuit in the stereo.

You could have weak charging/starting system connections connections due to rotted/corroded battery/alternator/starter/engine block ground posts/connections or the cables could even be rotted internally that connect up to those connections.Again weak or poor grounds or loose connections cause a drain on the whole system that could over draw some circuits causing them to temporally loose their proper operating voltage/power.