Anything developmental related - go by corrected - so head holding, smiling, toys etc
Everything else - actual ie injections, weaning etc.
You only correct for developmental purposes.
I can't really remember when Jamie started to be interested in toys - maybe 3 months for bashing things on his bouncy chair and around a month old for holding his head up. He did everything at the average time corrected pretty much although the time flies feels like I can't really remember!