DS1320 Real-time Clock

Some microcontroller applications require a functionality called timekeeping, which is
often within the realms of ‘software only’. Software timekeeping is not particularly complex
as examples showing ‘how it’s done’ are available on the Internet for practically any microcontroller. Unfortunately the method is less suitable for applications requiring higher accuracy; where the power consumption in the standby state has to be minimised; or in the (rare) case of insufficient memory space being available for the timekeeping code.

Datasheet:
www.pdfserv.maxim-ic.com/en/ds/DS1302.pdf
Programming example (in MBASIC)
www.basicmicro.com/downloads/docs/DS1302RTC.pdf