Справка LibreOffice 25.2
Calculates the difference between two date values. Returns the number of days between the two days. May return a fraction of a day when the arguments include time parts.
ДНИ(Дата2; Дата1)
Дата1: начальная дата, Дата2: конечная дата. Если Дата2 более ранняя дата, чем Дата1, результатом будет отрицательное число.
=DAYS(TODAY();"2010-01-01")) returns the number of days from January 1, 2010 until today.
=DAYS("1990-10-10";"1980-10-10") returns 3652 days.
=DAYS(NOW(); TODAY()) returns a fraction of the current day.