delay() is inaccurate, averaging 500us too short.

Since delay(n) waits for n ticks of the millisecond counter, any time already elapsed in the "current" millisecond isn't taken into account.

This was fixed a long time ago for the AVR, but it didn't make it into SAMD/SAM

https://github.com/arduino/Arduino/issues/237

https://github.com/arduino/ArduinoCore-avr/commit/67c0a1995aace2953e481ef6183b1b91e04942fc#diff-ab397febf179e1982f4ee0758df4c982

Start at the SAM/SAMD implementation of delay(n) and compare its millisecond-counter behavior with the linked AVR issue and commit. Done means elapsed time in the current millisecond is accounted for so delay(n) no longer averages about 500us too short.

Written by the indexing model from the issue text.