have you ever considered a RTC chip for this purpose?
I see the clock is built from conventional logic.
it would be difficult to load the clock registers, and once you use a microcontroller, you don't need the explicit logic anymore.
but also do you really need a 6v lead battery to power the CMOS chips?
i made a CMOS LCD clock once, using the 4000 series, and overall current was 500uA.
it worked from 4x AA cells for 6 months.
it is NP to charge NiMH batteries with 1/20 or 1/30 current permanently, maybe capacity will drop over time, but they will not take damage.
by the way, good web page. there should be more such pages.
do you have a backup plan when GEOCITIES is closing later on this year?
I show you one of my PIC clocks, powered by small solar cells, and 2.4 volts rechargeable battery.
it can work 24/7, the LED display is pulsed polyphase, 3 different pulsing schemes at once, and after a short moment, brightness is also dimmed.
works at 1mA average current!
it is hard to believe these small cells can power LED display 24/7 but it is true.
what I mean is, my suggestion is to pulse the display, in case power is lost, to reduce power consumption.
this could also be done with conventional CMOS logic.
maybe you do not want to use a smaller battery anyway.