A Question about the mdate feature

If one types ‘mdate:0d’, this supposedly references ‘zero days’. (I love this feature). Does this relate to the local time? IOW, does ‘0d’ mean ‘everything modified since midnight local time’?

Yes, all of the day search intervals start from 00:00:01 local time. So 0d refers to anything modified today and 1d is everything since midnight yesterday, and so on.