ILR - 180 days rule - what days do you count?

When calculating the days you have been absent from the UK on holiday etc. do you count the day you leave and return?

Is there any guidance on this?

Also a number of times I have been away I haven't been give a stamp when I come back into the UK - therefore I don't remember the exact date I came back.

Is there any way they can check when I did enter the UK - has this ever happened to anyone?

Thanks.

you don’t need to count arrival and departure days.