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copied!<p>Shameless plug:</p> <p><strong>Use a better date and time API</strong></p> <p>The built-in .NET date and time libraries are horribly hard to use properly. They <em>do</em> let you do everything you need, but you can't <em>express</em> yourself clearly through the type system. <a href="http://noda-time.blogspot.com/2011/08/what-wrong-with-datetime-anyway.html" rel="noreferrer"><code>DateTime</code> is a mess</a>, <code>DateTimeOffset</code> may lull you into thinking you're actually preserving the time zone information when you're not, and <code>TimeZoneInfo</code> doesn't force you to think about everything you ought to be considering.</p> <p>None of these provide a nice way of saying "just a time of day" or "just a date", nor do they make a clear distinction between "local time" and "time in a particular time zone". And if you want to use a calendar other than the Gregorian one, you need to go through the <code>Calendar</code> class the whole time.</p> <p>All of this is why I'm building <a href="http://noda-time.googlecode.com" rel="noreferrer">Noda Time</a> - an alternative date and time library built on a port of the <a href="http://joda-time.sf.net" rel="noreferrer">Joda Time</a> "engine" but with a new (and leaner) API on top.</p> <p>Some points you may want to think about, which are easy to miss if you're not aware of them:</p> <ul> <li>Mapping a local date/time to one in a particular time zone isn't as simple as you might think. A specific local date/time might occur once, twice (ambiguity) or zero times (it's skipped) due to daylight saving transitions</li> <li>Time zones vary historically - more than <code>TimeZoneInfo</code> is generally willing to reveal, frankly. (It doesn't support a time zone whose idea of "standard time" changes over time, or which goes into permanent daylight saving time.)</li> <li>Even with the zoneinfo database, time zone IDs aren't necessarily stable. (CLDR addresses this; something I'm hoping to support in Noda Time eventually.)</li> <li>Textual representations of dates and times are a nightmare, not just in terms of ordering, but date separators, time separators, and odd things like genitive month names</li> <li>The start of the day isn't always midnight - in Brazil, for example, the spring daylight saving transition moves the wall clock from 11:59:59pm to 1am</li> <li>In some cases (well, one that I know about) a time zone can force a whole day to be skipped - December 30th 2011 didn't occur in Samoa! I suspect most developers can probably ignore this one, but...</li> <li>If you're going to use a calendar other than the Gregorian one, be careful and make sure you really know how you expect it to behave.</li> </ul> <p>As far as specific development practices:</p> <ul> <li><strong>Think about what you're really trying to represent.</strong> I expect the core benefit of Noda Time to be forcing developers to choose between various different types to represent their data. Get that right, and everything else is simpler.</li> <li>Unit test everything you can think of. That will depend on exactly what your system does, of course, but <em>particularly</em> consider different time zones, what happens across daylight saving transitions, and of course leap years.</li> <li>I'd advise injecting a "clock-like interface" - a service for telling the current time - rather than explicitly calling <code>DateTime.Now</code> or <code>DateTime.UtcNow</code>; it makes it easier (feasible!) to unit test</li> <li>If you're performing multiple operations with "now", obtain that date/time <em>once</em> and remember it, rather than repeatedly requesting "now" - otherwise the value could change in unfortunate ways between the calls.</li> <li>"Do everything in UTC" isn't always the answer either - if I want to know "when exactly does 'two weeks from now' occur in my local time zone?" then I need to store the <em>local</em> date/time as well as the time zone.</li> </ul>

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