Is it necessary to put a space before and after the symbol en dash (–) put between two complete dates?
January 1, 2013 – January 1, 2014
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While spaces are not normally used with en dashes, and exception is often made in this case because without a space it can turn "2013–January" into a single visual unit, and so more closely bind these parts of the separate dates, than they are to the rest of the date they actually belong to.
We would not use spaces with partial dates like "June–August 2012", because it doesn't suffer from this problem.
With full dates, you might also decide to prefer to over a dash. Similarly, while the en dash is also used with numerical ranges, one should favour to if it could be mistaken for a hyphen minus (– and - or even the more typographically precise − aren't exactly easy to tell apart [en-dash, hyphen-minus and minus respectively]), so "12–14 June 2013" is fine, but "2m to 3m" should be favoured over "2–3m". Bringing this back to dates, "2012-01-01 to 2013-01-01" is clear, "2012-01-01 – 2013-01-01" just about bearable, and "2012-01-01–2013-01-01" rather nasty.