Assuming that the RTC is programmed at time S, then at time T the RTC is showing value R. Each of these time values is the UNIX Epoch time are represented in terms of seconds with respect to a fixed reference, which for UNIX is 00:00:00 on 1/1/1970.
If C and F are defined as:
C = Value of the calibration register (in the seconds calibration field).
F = Value of the calibration register (in the fractional calibration field).
and fractional calibration is enabled, then the actual crystal oscillator frequency is defined in This Equation.
CNEW = Int(Xf) – 1
FNEW = Round((Xf – Int(Xf))×16) – 1