The DAY([unixtime]) function returns the day of the month (1-31), either the current day, or the day of the specified unixtime.
DAY() === the current day DAY( unixtime ) === the day of the month of the unix time
Unix Time (also known as POSIX time or Epoch time) is a system for describing instants in time, defined as the number of seconds that have elapsed since 00:00:00 Coordinated Universal Time (UTC), Thursday, 1 January 1970
Example: 1451492206 – the corresponding ISO 8601 formatted date is 2015-12-30T16:16:46Z