Check the voltage at your battery with the car off. Should be at least 12.5 Volts. Turn on your car and check voltage at battery with car running. Should be around 13.8 volts. If it is less than 12 volts with the car off, your battery is getting weak.

Your clock is getting old, and the push button that is used to change the time is sticking. When you disconnected the battery, your clock stopped. When you reconnected it you had to change to the present time which meant you pushed in the knob and turned it to the correct time. When you released the knob, it did not come out all of the way. Push the button in,and then change the time and then pull the button out all of the way. I sometimes have to do this 5 or 6 times on both my 85 and 86 S class cars. It always happens to me in the spring when we spring forward and in the fall when we fall back and I have to change my clock time.
Just fool around with the knob a little and it should start to work correctly.

__________________
John
1986 300sdl
1985 380se