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copied!<p>First, that <code>leap</code> function feels overly complicated; you don't need to do both dates in one function call, and I'm sure that can be written more succinctly so that it is more obviously correct. Here's a version I've got laying around that isn't <em>succinct</em> but I'm confident it is easy to check the logic:</p> <pre><code>int is_leap_year(int year) { if (year % 400 == 0) { return 1; } else if (year % 100 == 0) { return 0; } else if (year % 4 == 0) { return 1; } else { return 0; } } </code></pre> <p>You could call it like this:</p> <pre><code>int year1, year2, leap1, leap2; year1 = get_input(); year2 = get_input(); leap1 = is_leap_year(year1); leap2 = is_leap_year(year2); </code></pre> <p>No pointers and significantly less code duplication. Yes, I know that <code>is_leap_year()</code> can be reduced to a single <code>if(...)</code> statement, but this is easy for me to read.</p> <p>Second, I think you're got a mismatch between 0-indexed arrays and 1-indexed human months:</p> <pre><code> if(*month1 < 1 || *month1 > 12) </code></pre> <p>vs</p> <pre><code> int daysPerMonth[] = {31,28,31,30,31,30,31,31,30,31,30,31}; </code></pre> <p>Third, I think that days per month can be calculated slightly nicer:</p> <pre><code>int days_in_month(int month, int year) { int leap = is_leap_year(year); /* J F M A M J J A S O N D */ int days[2][12] = {{31, 28, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31}, {31, 29, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31}}; if (month < 0 || month > 11 || year < 1753) return -1; return days[leap][month]; } </code></pre> <p>Here, I assume January is 0; you would need to force the rest of the code to match. (I learned this double-array trick from <a href="http://en.wikipedia.org/wiki/The_Elements_of_Programming_Style">The Elements of Programming Style</a> (<a href="http://books.google.com/books?ei=jzeXT--_OYeYiAKJjuX6Dw&id=-_QmAAAAMAAJ&dq=%22the+elements+of+programming+style%22&q=leap+year#search_anchor">page 54</a>).) The best part of using a routine like this is that it removes the leap condition from the difference calculation.</p> <p>Fourth, you're indexing arrays outside their bounds:</p> <pre><code> for(i = month1 + 1; i <= 12; i++) { if(leap1 == 1) total += daysPerMonthLeap[i]; </code></pre> <p>This is just another instance of the problem with 0-indexed arrays and 1-indexed months -- but be sure that you fix <em>this</em>, too, when you fix the months.</p> <p>I have a fear that I haven't yet found all the issues -- you may find it easier to <em>sort</em> the first and the second date after input and remove all that validation code -- and then use names <code>before</code> and <code>after</code> or something to give names that are easier to think through in the complicated core of the calculation.</p>

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