GNU date's -d switch accepts a large variety of time formats.  So far,
the best documentation I've found for this option is

    info coreutils 'Date input formats'

The examples given in this section are useful, but I would very much
like it to be augmented with an ABNF[0] that describes concisely and
precisely describes how the -d argument is parsed.

[0] http://tools.ietf.org/html/rfc5234