[TW-1618] Taskwarrior incorrectly interprets an ISO-8601 date containing only a year

_Jeremy John Reeder on 2015-05-13T21:42:22Z says:_

"2017" is a valid ISO-8601 date, although it's not listed among [Taskwarrior's supported date formats|http://taskwarrior.org/docs/dates.html]. Taskwarrior should either accept that date with the correct ISO-8601 interpretation or produce an error message. I'd prefer correct interpretation over an error message, but I'd accept either. Instead, Taskwarrior silently accepts the date with an unreasonable interpretation. That is, when I set the "wait" date for a task to "2017", it is interpreted as the crazy "1969-12-31T17:33:37" rather than the sane "2017-01-01T00:00:00" that I actually mean.