А вот этого разве мало? (а это как раз оно самое...):
3.1 RTC.1: The Real Time Clock (RTC) does not work reliably within the
temperature specification
Introduction:
The RTC is a set of counters for measuring time when system power is on, and optionally
when it is off. The RTC is clocked by a separate 32 kHz oscillator that produces a 1 Hz
internal time reference. The RTC is powered by its own power supply pin, VBAT, which
can be connected to a battery, externally tied to a 3 V supply, or left floating. The RTC can
operate over temperature range from 40 C to 85 C.
Problem:
The RTC does not work reliably within the temperature specification.
Work-around:
None.