I am not sure that I understand your question. All of the standard CPUs come with a real time clock.
Precision of real-time clock - 120 seconds/month
Real-time clock hold time - In general 7 days, or min. 6 days when 25 °C (Maintenance free super capacitor)
You can also get a 'Single Board' - SB BA01 - Battery module - It supports the generic CR1025 cell (battery), which can drive the clock for about 1 year.