The mishna describes a case where a man vows twice to be a nazarite
at times that overlap. How is this to be resolved -- does he serve
consecutive terms, does he interrupt one for the other, or does one
of them become void? If he says "I intend to be a nazarite now
and a nazarite when I have a son", then if he has a son before the
first nazarite term ends, he finishes that one first and then
commences the second.
If instead he reverses them and says "I intend to be a nazarite when
I have a son, and to be a nazarite now", he begins serving as a
nazarite ("now") and interrupts it when his son is born to serve
the other. (13b-14a)
The gemara does not here spell it out, but it appears
that it is the order of specifying the conditions that matters.
Vows are not commutative.
Also, since interrupting one period of nazirut to serve another is
complicated, one might reasonably draw this lesson: pay attention to
due dates. :-)