DAY
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.
DAY(Number)
Number is the internal date number.
When entering dates as part of formulas, slashes or dashes used as date separators are interpreted as arithmetic operators. Therefore, dates entered in this format are not recognized as dates and result in erroneous calculations. To keep dates from being interpreted as parts of formulas use the DATE function, for example, DATE(1954;7;20), or place the date in quotation marks and use the ISO 8601 notation, for example, "1954-07-20". Avoid using locale dependent date formats such as "07/20/54", the calculation may produce errors if the document is loaded under different locale settings.
Unambiguous conversion is possible for ISO 8601 dates and times in their extended formats with separators. If a #VALUE! error occurs, then unselect Generate #VALUE! error in Collabora Office - PreferencesTools - Options - Collabora Office Calc - Formula, button Details... in section "Detailed Calculation Settings", Conversion from text to number list box.
=DAY(1) returns 31 (since Collabora Office 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).