This is a small DS1302 PCB with on-board oscillator. This means that if you need a real-time clock (RTC) you don't have to worry about problems with crystals, capacitors and also the layout of ground layers. This is done here for you. On the top of the PCB you have the pin names printed for ease of use if put on a breadboard. The DS1302 has a trickle charge feature that can be used to charge a goldcap so the RTC doesn't require a battery.
The schematic is pretty simple as it consists of only four pars: a DS1302ZN+, a ASH7KW 32.768 kHz crystal and a optional 100 nF cap.
For the accuracy, if we calculate worst case from the datasheet we get:
32,768 Hz with +/-20 ppm result in 51.84 seconds offset for 30 days.
This means we will be ~10 minutes off in a year in the worst case.
The DS1302 is supported by a few Arduino libraries and only requires three pins on the microcontroller to operate. With the current design you can upgrade for example the 3 displays alarm-clock to give it a more stable timebase. For the update procedure you can have a look at the article about the clock on the Labs. The final PCB now look like this:
Inspiration of how to add this RTC to your own project can be taken from the software of the 3 displays alarm-clock.