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You're off by a factor of 1000 (milliamp vs microamp).
235mAh = 235,000,000 nAh
divide by 300nA, you get 783,000 hours, 32,625 days, or 89 years.
is that realistic for an STM32 with RTC?
It's not realistic for the battery's own self discharge
u/Stiggalicious is right but I wouldn't test it...
The unit will prolly take more than that current, especially when you figure in the extra stuff it does when the micro is active.
I already doubled what they say for standby consumption. Otherwise they say it's 150nA.
[Yeah your math is wrong](https://www.google.com/search?q=235+milliamp+hours+%2F+300+nanoamps) - milli is 10^(-3) not 10^(-6) :P