Last modified: 2011-04-14 15:14:05 UTC
The timestamp parameters in the API (the one I tested with was wlbegin in the watchlist querying module) do not accept all ISO 8601 dates, only a subset. For example, "2008-06-06T19:00:00Z" is accepted while "2008-06-06T14:00:00-04:00" is not.
The second date should have been "2008-06-06T15:00:00-04:00" in order to be equivalent to the first date.
Technically, this is not a bug in the API, but in the MediaWiki core. The problem is in wfTimestamp() in GlobalFunctions.php , which is used by e.g. Special:BlockIP too.