Getting the Nth Occurrence Of A Day Of The Week For A Given Month

<cough><cough>cflib</cough></cough>

http://www.cflib.org/udf/GetNthOccOfDayInMonth

From 2001. ;)

One extra thing you may want to add is LAST (I just wrote a date utils for Flex that includes this...mine had an even weirder name dayOfWeekIterationOfMonth :) ). This gets the last occurrence of that day of the week for the month. A little easier in Flex to do this as you can assign a constant to that value, and have people pass in FlexDateUtils.LAST as the occurrence to get, but more logical than having to pass in 7 (for e.g.) to get the last occurrence of a day of the week.

CFLib ftw again: http://www.cflib.org/udf/GetLastOccOfDayInMonth

Bah. I say keep on coding before you look stuff up. Some might tell you that your time is worth more than that, but not me. Anything that stretches the brain is a good thing.

(But do look it up when you are done and make sure your solution doesn't completely suck, of course. To err is human.)

@Rick: Yes, that. Absent a time constraint, I like poking around at a problem first. It's almost like homework, with the internet being the answers in the back of the book.

Of course, while many books are pretty solid, some can have sketchy solutions ... don't be afraid to question something if it doesn't match what you expected.

You probably already know this, but when (and who observes) daylight time savings occurs changes over time (most recent change is just a couple of years old). Any algorithm would need to have an effective (start/end) date if were to be used over a long period of time.

I didn't realize this was to account for DST (I finally read the whole post :) ). I actually wrote something to account for DST also. I made a DaylightSavingTimeUS class to handle the changes for the US.

I created a DSTStart method and DSTEnd method that accepts a date (or could be a year). These are the public methods, then I have 2 private methods that account for any differences in the DST prior to the current DST start and end. I called them 'DSTStartBefore2007' which is probably not the best name for them if/when they change in the future, but you get the idea :) Then whenever it changes, I just add the new changes to the "before" methods, and anyone calling the DSTStart or DSTEnd won't notice any difference.

I created my Flex version of this to try to calculate US Federal Holidays, which also need the "get X day of the week for X month".

As long as we're talking about it ... certainly DST for a global application is a pain, but even within the US there are challenges. Not everyone observes daylight saving time, and sometimes there are changes to that list that are not necessarily in sync with the changes to DST itself.

This is why you have the Indiana and Arizona time zones in Windows et al: currently only Arizona and Hawaii do not observe DST. Indiana began observing it in 2006. (My previous employer did a lot of phone surveys, so before 2006, we had semi-annual fun adjusting all appointments over the weekend and then explaining to our clients for the next month or so why our time was different.)

It's not a complication if you're displaying application time, but if you're trying to display local time, it can be tricky (which is why some apps simply ask for East/Central/Mountain/Pacific and DST on/off: fine for continental US, inadequate otherwise), and historical local time can be a nightmare.

I wrote this algorithm in PHP. It can probably pretty easily be converted to any language. Read the comments that I inserted in the code to get an idea of how the algorithm works.

This will give you any occurrence of a day of week. For example, the third Tuesday of the month, the last Friday of the month, the second Monday of the month… you get the picture.

$DOW = day of the week (0 = Sunday, 6 = Saturday).
$X = occurence (1 = first, 2 = third, etc..). If the given month does not have the occurrence, then it will return the last. For example, if you ask for the 7th occurrence of Friday, it will return the last Friday of the month.
$M = month
$Y = year

Example, get_Xth_DOW(2,3,7,2009) will return the third Tuesday of July 2009. It just returns the day. If you want the date you can just do
$D = get_Xth_DOW($DOW,$X,$M,$Y);
echo $M . "/" . $D . "/" . $Y;

The algorithm (bottom of post) is quick and simple.

It first gets the first occurrence. It can then add multiples of 7 to get the Xth occurrence.

function get_Xth_DOW($DOW,$X,$M,$Y) {
# Start at the first day of the month
$d = 1;

# Get the day of week for the first day of the month
# date() returns 0 = Sunday, through 6 = Saturday
$firstDOW = date('w',mktime(0,0,0,$M,1,$Y));

# Get the difference in the number of days between
# the dow of the first of the month and the dow that
# you are looking for.
$DOW_offset = $firstDOW - $DOW;

# Adjust the date (d) to get to the first occurrence
if($DOW_offset > 0) { $d += (7 - $DOW_offset); }
else if ($DOW_offset < 0) { $d += -1*$DOW_offset; }

# Add multiples of 7 to the first occurrence of the dow
# to get to the Xth occurrence
$add = 7*($X - 1);
$d = $d + $add;

# If you went over the number of days in the month,
# then it is time to backtrack to the last occurrence
# of dow.
# numDays is the number of days in the current Month and Year
$numDays = date('t',mktime(0,0,0,$M,1,$Y));
while($d > $numDays) { $d -= 7; }

# return the day.
return $d;
}

@Joe Dundas, thanks for your code. It's been very helpful to me. I'm working on a slight variation and I wonder if you have any ideas. What I'd like to do is determine what occurrence *today* is (e.g. today, March 10th, is the 2nd Tuesday of the month). So, I'd like to turn this logic around just a little bit and take today's day/date and return what nth occurrence it is in this month.

Ideas? Much appreciated.

Hi John,

Take the ceiling of $D/7. Where $D is the day of the month.

For example,

for 2/11/2009.
$D = 11;
$occurrence = ceil($D/7) = ceil(1.57) = 2