Explanation for micros function
If the timer 0 has overflowed (TIFR0 &amp; _BV(TOV0)) and there is at least a count of 1 in the timer count register (t &amp; 255) then increment the local copy of the overflow count.

That overflow count is used later on to add an upper 8 bits to what is basically an 8 bit counter to turn it into a 16 bit one ((m &lt;&lt; 8) + t).