Crystal Oscillator Accuracy

I don't have your answer but this ST Application note is deeply linked to crystals on STM32 family and might help you.
There is also a discussion on mbed which is argueing about a 10% accuracy.
See the last post it seems to be written in the specifications. I did not found that particular spec on st.com :(
This reference manual says 'around 32 kHz' which is not a good sign.
However, the chapters about the RTC seems more optimistic.

17.2 RTC main features
The RTC unit main features are the following (see Figure 159: RTC block diagram):
• Calendar with subseconds, seconds, minutes, hours (12 or 24 format), day (day of
week), date (day of month), month, and year.
• Daylight saving compensation programmable by software.
• Two programmable alarms with interrupt function. The alarms can be triggered by any
combination of the calendar fields.
• Automatic wakeup unit generating a periodic flag that triggers an automatic wakeup
interrupt.
• Reference clock detection: a more precise second source clock (50 or 60 Hz) can be
used to enhance the calendar precision.
• Accurate synchronization with an external clock using the subsecond shift feature.
• Maskable interrupts/events:
– Alarm A
– Alarm B
– Wakeup interrupt
– Timestamp
– Tamper detection
• Digital calibration circuit (periodic counter correction)
– 5 ppm accuracy
– 0.95 ppm accuracy, obtained in a calibration window of several seconds
...

Frankly I totally lost just from this document.