Almost C-IoT products are using PCH RTC, the RTC deviation definition needing to following Intel PCH RTC specification.
Following link is Intel RTC definition, and Intel notes RTC deviation definition is 1.728 sec/day at battery low power status.
Intel® I/O Controller Hub (Intel® ICH) / Platform Controller Hub (PCH) Family Real Time Clock (RTC):
Chapter 3.3.1 (P15)
The RTC accuracy can be affected by the voltage of battery. In general, when the battery voltage decays, the RTC accuracy also decreases. High accuracy (less than ±10 ppm which does not include Crystal’s tolerance) can be obtained when VccRTC is larger than the min voltage specification.
For example: 20 ppm is equivalent with 1.728 sec error in one day.PPM = ((Fo – 32.7680) / 32.768) * 10^61 day = 24 hours/day * 60 min/hour * 60 sec/min = 86400 sec20 ppm * 1 day (86400 sec) = (20 * 86400) / 1000000 = 1.728 sec