Monthly milestones

Peopy

You know those monthly milestone pictures everyone posts on fb, IG, etc? Well do they consider “1 month” 4 weeks from the birth date or the same date of the next month? For example if baby was born dec 01 would 1 month be dec 29 or Jan 01?

I realize I’m putting too much thought into this but I’m just wondering what everyone else does lol.