ISO 8601-2:2019: Extensions defines sub-year groups exactly as you are asking for.
These are defined in § 4.8 Sub-year Groupings. The sub-year groupings are an extension of the year-month format. (e.g. YYYY-XX
) where XX
= 01
- 12
to represent January - December.
XX
= 21
, 22
, 23
, 24
are used to represent Spring, Summer, Autumn, and Winter respectively.
Although, you mentioned June to August in your question so you might be referring specifically to the northern hemisphere Summer. The northern hemisphere specific seasons are 25
, 26
, 27
, 28
respectively.
For completeness 29
, 30
, 31
, 32
refer to the seasons in the southern hemisphere (Spring - Winter again).
It's also possible to refer to the four quarters (33
, 34
, 35
, 36
); three thirds (37
, 38
, 39
); and two halves (40
, 41
) of the year.
So to answer your question. "Summer 2011 (Northern Hemisphere)" would be represented as:
2011-26