<p>It's not X days, it's 5 minutes...</p>

<p>(In principle, this question belongs on meta.SO, but I think it's worth having an answer here that doesn't require people to go looking over there.)</p>

<p>As far as I can tell, <a href="http://meta.stackexchange.com/a/18046/130540">this answer of Jeff's</a> (one of the SO founders, though no longer part of the company) is still the official reason: they don't want people to downvote a post to make it look bad, post their own answer (which will look better because it's not -1), then later take the downvote away.</p>

<p>There are a <a href="http://meta.stackexchange.com/questions/80762/why-do-votes-get-locked">number</a> of <a href="http://meta.stackexchange.com/questions/6250/vote-too-old-to-be-changed-unless-post-is-edited">questions</a> about <a href="http://meta.stackexchange.com/questions/94567/you-last-voted-on-this-answer-6-mins-ago-locked-too-short">this</a> on meta.SO, but despite some good arguments against locking votes after 5 minutes, SO Inc. thinks it's fine the way it is, so it's not going to change.</p>

<p>There is a laborious workaround: edit the post then change your vote.  But that only works if you have enough rep to edit w/out your edit being reviewed.</p>

<p>So basically, don't worry about it.  Since Iain's answer is the only one with a downvote, it's obvious that that's the vote you'd like to change.  But it's just one downvote and since he gave an answer you'd ruled out your downvote it arguably appropriate and - unlike some SE sites - I don't think SF has people who go nuts about every single downvote.</p>

<p>TL;DR: Don't worry about it, just move on.</p>