As far as I understand, length is a property of the array that remains constant. So it is a simple property access to do "this.length". It is not a method, nothing is being calculated.

edit: But thank you for at least explaning the logic behind the usage!

The original format is a small optimisation, the length of the array is calculated once up front rather than on every loop iteration.

It's not necessary to say "len = this.length". The more common way to write for loops is simply:

for(var i = 0; i < this.length; ++i)

Per the description, startDate and endDate are already Date objects.