[Google ngrams]
I'd like to know what are the grammatical differences between both structures, as well as one or the other predominates for days and weeks.

Asked
Active
Viewed 504 times
1
GJC
- 2,491
-
Related: https://english.stackexchange.com/questions/252173/indefinite-articles-used-with-plural-nouns-it-was-an-amazing-two-days – herisson Sep 25 '20 at 18:16
-
I wonder if this has to do with preserving the construction "two weeks notice." One would say "an additional two weeks notice" but not "two additional weeks notice." – d_b Sep 25 '20 at 18:20
-
If you swap 'three' for 'two' in all these phrases, you get graphs that are far closer. And 'an additional four days' has outperformed 'four additional days' on occasion. – Edwin Ashworth Sep 25 '20 at 18:26
-
@d_b For me it's uncountable, either two weeks' notice or two-week notice. Yet I just found two-weeks' notice https://www.wordreference.com/definition/notice – GJC Sep 25 '20 at 18:34
-
@EdwinAshworth any reason why further behaves differently? https://i.imgur.com/v0bmvO1.jpeg – GJC Sep 27 '20 at 08:46
-
It's simple writer's choice. Which "sounds" best depends on other words in the context. – Hot Licks Sep 27 '20 at 12:28