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https://stackoverflow.com/questions/325933/determine-whether-two-date-ranges-overlap

accepted
(StartA <= EndB) and (EndA >= StartB)

Proof:
Let ConditionA Mean that DateRange A Completely After DateRange B
_                        |---- DateRange A ------| 
|---Date Range B -----|                           _
(True if StartA > EndB)

Let ConditionB Mean that DateRange A is Completely Before DateRange B
|---- DateRange A -----|                       _ 
 _                          |---Date Range B ----|
(True if EndA < StartB)

Then Overlap exists if Neither A Nor B is true -
(If one range is neither completely after the other,
nor completely before the other, then they must overlap.)

Now one of De Morgan's laws says that:

Not (A Or B) <=>  Not A And Not B

Which translates to: (StartA <= EndB)  and  (EndA >= StartB)