*Socialmetrica
periodsSince: startingDate until: endingDate
	| borders subperiodDuration |
	subperiodDuration := (endingDate - startingDate) / self.
	borders := OrderedCollection new.
	borders add: startingDate.
	1 to: self do: [ :i | | ending |
		ending := startingDate + (subperiodDuration * i).
		borders add:  ending.
	].
	^ borders