If you think about how we often say the time within an hour as "15 past X" and "15 til X," doing the same with days in the month doesn't seem as exotic.
Yeah, those guys.
My missus just shakes her head when I use the phrases such as "fem på halv" as in "five to half" which means 25 minutes past the current hour, or "ti over halv" ("ten past half") as in "twenty to the hour"...