My experience with internal RC oscillator:

By tuning you can get a reasonable accuracy of about +/- 1% for a given voltage and temperature. So the frequency may be "good enough" for UART if (and only if) voltage *and* temperature stay constant (UART serial requires timing to within +/-2% or so).

I mean +/- minute in a month or so.

So let's calculate how many minutes it may be off within one month:

60 * 24 * 30 = 43200 minutes per month

1% of 43200 = 432

So it may be +/- 432 minutes off within a month (that is about 7 hours).

Way too much...

But even with a crystal (50 ppm) you may not meet your needs.

I would recommend a real time clock: DS3231 (accuracy +/- some seconds per year).