I2C is a bus, it's sole purpose is to connect multiple devices together (1 master, multiple slaves)

This is the way I'll go for my project, I think it is a cheap way to get a correct precision of a millisecond :)

Thank you for the answer :) ; gps utc time sync might also be a good option for precise time syncronisation

@per1234 the "faster than expected" was a mistake from me, I belived that the serial transmited data faster than it should, but I was wrong. Btw thanks to everyone trying to help me :)

@Majenko If I understand correctly, I have to add 2 more bits to each byte to get an idea of the transfer time, that would do 7.4 * 35 * (8+2) ~ 2.6ms offset

@Majenko I've done a simple echo programm that reads from serial and returns the message at 300bauds So, according to wikipedia, it takes ~3ms to transmit a single bit at 300 bauds, if I transmit 22 characters, the time will be 2*22*8*3 = 1056ms or ~1s to send and read the data back (that's what I get by testing it, so I was wrong in my question sry for that) In my case I have 35 characters to transmit ; 7.4 micro seconds per bit at 57600 bauds ; so the time taken to send will be 7.4 * 35 * 8 = 2072 so that's 2ms offset. Sorry for asking a question that I was able to answer by myself...

@Majenko yes, that's what I'm trying to ask, sorry for not being clear.

I am using an arduino Uno or Nano with an ATMega238P