Returns the day of given date value. The day is returned as an integer between 1 and 31. You can also enter a negative date/time value.
Number, as a time value, is a decimal, for which the day is to be returned.
DAY(1) returns 31 (since LibreOffice starts counting at zero from December 30, 1899)
DAY(NOW()) returns the current day.
=DAY(C4) returns 5 if you enter 1901-08-05 in cell C4 (the date value might get formatted differently after you press Enter).