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    <p>This was harder to solve than expected, due to the problems caused by leap years and people who's birthdays are actually on a leap day.</p> <p>Finally, I remembered I had already a method that calculates someone's age in years accurately, accounting for leap years and birthdays that occur on a leap year.</p> <p>Given such a method, it's trivial to determine if someone has a birthday give a range of dates:</p> <p>First, calculate their age one day <em>BEFORE</em> the start of the range. Next, calculate their age at the end of the range.</p> <p>If the calculated ages differ, then they must have had a birthday in the specified range.</p> <p>Putting it all together:</p> <pre><code>public IEnumerable&lt;User&gt; UsersByBirthday(IEnumerable&lt;User&gt; users, DateTime from, DateTime to) { if (to &lt; from) throw new ArgumentException("'from' must be at or before 'to'."); return users.Where(user =&gt; IsBirthdayInRange(user.BirthDate, from, to)); } public bool IsBirthdayInRange(DateTime birthday, DateTime from, DateTime to) { if (to &lt; from) throw new ArgumentException("'from' must be at or before 'to'."); if (birthday &gt; from) throw new ArgumentException("'from' must be after 'birthday'"); return AgeInYears(birthday, from.AddDays(-1)) &lt; AgeInYears(birthday, to); } /// &lt;summary&gt;Returns a person's age in years, accounting for leap years.&lt;/summary&gt; public static int AgeInYears(DateTime birthday, DateTime today) { // See here for why this works: // http://social.msdn.microsoft.com/Forums/en-US/ba4a98af-aab3-4c59-bdee-611334e502f2/calculate-age-with-a-single-line-of-code-c return ((today.Year - birthday.Year) * 372 + (today.Month - birthday.Month) * 31 + (today.Day - birthday.Day)) / 372; } </code></pre> <p>Here's a little test for a leap day birthday:</p> <pre><code>DateTime test = new DateTime(2004, 2, 29); // Leap day. var start = new DateTime(2005, 3, 1); var end = new DateTime(2005, 6, 7); Console.WriteLine(IsBirthdayInRange(test, start, end)); </code></pre> <hr> <p>Using your <code>_unit.User.Load()</code>:</p> <pre><code>public IEnumerable&lt;User&gt; LoadUsersByBirthday(DateTime from, DateTime into) { return _unit.User.Load(u =&gt; IsBirthdayInRange(u.Birthdate, from, into)); } </code></pre> <hr> <p>[EDIT]</p> <p>There are two possibilities for handling a leap day birthday in a non-leap year: Treat it as Feb 28th, or treat it as Mar 1st. </p> <p>The code above treats it as Mar 1st (as per UK regulations). If you want to parameterise the handling, you can add a <code>leapDayMapsToFeb28</code> parameter to <code>IsBirthdayInRange()</code> as follows:</p> <pre><code>public bool IsBirthdayInRange(DateTime birthday, DateTime from, DateTime to, bool leapDayMapsToFeb28) { if (to &lt; from) throw new ArgumentException("'from' must be at or before 'to'."); if (birthday &gt; from) throw new ArgumentException("'from' must be after 'birthday'"); if (leapDayMapsToFeb28 &amp;&amp; (birthday.Month == 2) &amp;&amp; (birthday.Day == 29) &amp;&amp; !DateTime.IsLeapYear(from.Year)) birthday = new DateTime(birthday.Year, birthday.Month, 28); return AgeInYears(birthday, from.AddDays(-1)) &lt; AgeInYears(birthday, to); } </code></pre>
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    1. POLoad users by birthday
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    1. USMatthew Watson
    1. USMatthew Watson
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    1. COYou was right about leap years, but I don't have to throw an exceptions, `LoadUsersByBirthday(DateTime from, DateTime into)` method has particular signature, where i must to give to the method only 2 dates, and method should returns users as was described before.
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    2. CO@Art713 Your question didn't define what _unit.User.Load() does, so I have provided a general solution which will work with any sequence of Users. You should easily be able to plug in your user sequence.
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    3. CO_unit.User.Load(predicate) just gives me users that satisfy the predicate
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