date -d '...' should support superset of POSIX 'at' dates

date -d '...' should support superset of POSIX 'at' dates

[Eric Blake]

On today's Austin Group meeting, the question of whether some form of 
'date -d "..."' should be specified.  While there is no way that POSIX 
would standardize everything that GNU date can parse, there was an idea 
that supporting '@seconds' since Epoch, plus all of the formats already 
required to be supported by the 'at' utility 
https://pubs.opengroup.org/onlinepubs/9699919799/utilities/at.html might 
be acceptable.

But that would imply that we want gnulib's date parser to be a superset 
of the grammar required by 'at'.  We are already most of the way there; 
required support dates such as '2pm + 1 week', '2pm next week', and 
'tomorrow' work; but we are lacking at least 'midnight'.  Thus, I'm 
opening this bug to remind us to add the remaining few items to our date 
grammar, so that we can parse everything 'at' is required to support, 
and in case POSIX does decide to standardize -d using the 'at' grammar 
as a starting point.

-- 
Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3226
Virtualization:  qemu.org | libvirt.org