There was a recent question on FB regarding the apostrophe in this construction:
…It was 2013-’14.
…It was 2013-14.
I wanted to look it up to make sure but was running out the door at the time — and then promptly forgot about it. Here is a post from today by Mary Neel with the reference that cites the rule.
Gregg 460. Check the note.
a. In a continuous sequence of ?gures connected by an en dash or a hyphen, the second ?gure may be expressed in abbreviated form. This style is used for sequences of page numbers or years when they occur quite frequently. In isolated cases, do not abbreviate. (See ¶217.)
1997–98 (OR: 1997–1998)
pages 110–12 (OR: pages 110–112)
2008–9 (OR: 2008–2009)
pages 101–2 (OR: pages 101–102)
NOTE: An apostrophe is used to signify the omission of numbers from a date that stands alone (for example, during the year of ’09). However, no apostrophe is used when an abbreviated number appears in a sequence of dates (2008–9).
Thank you, Mary Neel. And now we know that the apostrophe is unnecessary in this construction.