The difference between expires and Max-age

Both expires and max-age can be used to specify the expiration time of a document, but there are some minor differences

1.Expires has been defined in the http/1.0, cache-control:max-age in http/1.1 is defined, in order to backward compatibility, only use max-age enough;
2.Expires specifies an absolute expiration time (GMT format), which results in at least 2 issues 1) client and server time synchronization causes problems with configuration of expires 2) it is easy to forget the specific expiration time after the configuration, resulting in the arrival of the expiration of a surge phenomenon;

3.max-age specifies the time to live after the document has been accessed, which is a relative value (for example, 3600s), relative to the Request_time (Request time) recorded by the server the first time the document was requested.

The time specified by 4.Expires can be relative to the last access time (Atime) or modification time (MTime) of the file, whereas Max-age is relative to the document's request time (Atime)

5. In Apache, Max-age is calculated based on expires time Max-age = Expires-request_time: (MOD_EXPIRES.C)

429 expires = base + additional;
430 Apr_table_mergen (T, "Cache-control",
431 apr_psprintf (R->pool, "max-age=%" apr_time_t_fmt,
432 apr_time_sec (Expires-r->request_time)));

Note: If it is a,base=request_time,m words base=finfo.mtime.
Expires-request_time Get Max-age, if expires is based on a (that is, accesstime) set (the value after a is addtional), then the value of expires set is equal to the addtional value, But if expires is based on Mtime, then if the parameter behind M is less than the difference from the last modification time to the current time (say 2 hours ago, the file was modified (19:00:00), now set M3600 (now 21:00:00) and access, max-age= Expires-request_time = (finfo.mtime+additional)-request_time), the calculated max-age is a negative number (you can experiment to see this result):
Max-age = (19:00:00+3600/3600)-21:00:00 = -3600s

Thus, in Apache, Max-age is not just a relative atime time, if set to M, the relative is mtime.

The difference between expires and Max-age