This Real Time Clock addon was published as a DIY project in a special edition of the the German magazine: CPC Schneider International(Sonderheft, #3-1986)
Like most other homecomputers of the era, the Amstrad CPC did not include a Real Time Clock.
This project overcomes this limitation.
Technical specs
Based around: SUWA RTC-58321
Ports used: FBxx, on the device there is a jumper that determines the value of xx.
Commercial versions
There are or have been other projects with an own Real Time Clock solution: