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    <p>Pseudocode:</p> <pre><code>int julian = getDayOfYear(myDate) // Jan 1 = 1, Jan 2 = 2, etc... int dow = getDayOfWeek(myDate) // Sun = 0, Mon = 1, etc... int dowJan1 = getDayOfWeek("1/1/" + thisYear) // find out first of year's day // int badWeekNum = (julian / 7) + 1 // Get our week# (wrong! Don't use this) int weekNum = ((julian + 6) / 7) // probably better. CHECK THIS LINE. (See comments.) if (dow &lt; dowJan1) // adjust for being after Saturday of week #1 ++weekNum; return (weekNum) </code></pre> <p>To clarify, this algorithm assumes you number your weeks like this:</p> <pre><code>S M T W R F S 1 2 3 &lt;-- week #1 4 5 6 7 8 9 10 &lt;-- week #2 [etc.] </code></pre> <p>getDayOfWeek() and getDayOfYear() are standard date-object operations in most languages. If yours doesn't have them, you can count-forward from some known date (Jan 1, 1970 is a common one), after looking up to see what day of the week it was.</p> <p>If you're going to implement your own date counting routines, remember that years that are divisible by 100 are <strong>NOT</strong> leap years, unless they are also divisible by 400. So 1900 was not a leap year, but 2000 was. If you're going to work far back in time, you have to mess with Gregorian vs Julian calendars, etc., see <a href="http://en.wikipedia.org/wiki/Calendar_reform" rel="noreferrer">Wikipedia</a> for loads of info on that.</p> <p><a href="http://www.codeproject.com/KB/datetime/datetimedisc.aspx" rel="noreferrer">This link</a> talks about date/time functions in Windows/C++ in greater detail.</p>
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    1. POHow do I calculate the week number given a date?
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    1. POHow do I calculate the week number given a date?
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    1. COThis fits my simple needs perfectly, thanks
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      1. USBig GH
    2. COI'd agree it doesn't do what I asked for, but I realised I'd not been asking for what I really wanted! My definition of week no. was one I thought was "standard", but as it wasn't I was happy to change my definition. I think your answer might be the more correct one, but this was the one I used.
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    3. COUnless I'm mistaken, this isn't correct. Just take Jan 7th from the example: juilan = 7, dow = 3, dowJan1 = 4, weekNum = (7/7)+1 = 2. Then dow is less than dowJan1 so weekNum+1 = 3. But clearly Jan 7 is week 2.
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