Q:

Is 年月 (months and years) read 【としつき】or【ねんげつ】and is there a difference?

Is 年月 (months and years) read 【としつき】or【ねんげつ】?
Is there a difference?
I found it in the following sentence in WWWJDIC:
年月が過ぎるのは早い。

A:

There certainly is a difference in nuance and therefore, how the two readings are used in real life if not in their basic "dictionary" meaning.  Frankly, there would not exist two completely different readings if there were no difference between the two in the first place.
「ねんげつ」 tends to sound more formal, academic, technical, etc., which are the characteristics of on-reading words.
「としつき」 tends to sound more informal, personal, intuitive, emotional, poetic, etc., which are often the characteristics of kun-reading words.
In the sentence 「年月が過{す}ぎるのは早{はや}い。」, 「年月」 can be read either way.  It is the larger context from which the speaker/author decides which reading to use.
Other examples of these "word pairs":
・「ぼくじょう」vs.「まきば」 for 「牧場」 "ranch"
・「みょうにち」vs.「あした」 for 「明日」 "tomorrow"
・「ざいにん」vs.「つみびと」 for 「罪人」 "criminal"
・「しゅんかしゅうとう」vs.「はるなつあきふゆ」 for 「春夏秋冬」 "the four seasons"
Finally, a couple of kanji compounds where their meanings will actually change rather drastically depending on how they are read:
「人気」:
It means "popularity" when read 「にんき」, but it means "signs of life" when read 「ひとけ」.  Thus, the simple sentence 「X島は人気がない。」 can mean both:
1) "X Island is not popular (as a place to visit)." and
2) "（Almost）no one lives on X Island."
「色紙」:
When read 「しきし」, it refers to an extra-thick white-colored paper on which one asks a "famous" person to sign an autograph.
When read 「いろがみ」, it means "thin colored paper" used often in origami.

