pass by means to go past or near smth. com/pass+by or to overlook or disregard something to pass by difficult problems And it shuld be noted that pass by is usually means the same as pass So that you should write Four months have passed (by) since... Five years passed (by) ...
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The example is taken from the http://www.thefreedictionary.com/pass+by
A black limousine passed by when she looked out the window
So that pass on can be used when we are talking about time...
He passed on in 1978