Question

I have a what seems like simple date issue and I just can’t wrap my head around trying to get it efficiently… I basically need to get the previous months date for a specific day.

For example: If today is the 3rd Thursday of the month, I want to get the 3rd Thursday’s date of last month. Its important that its based of the number of the day…ie: First Monday, 4th Friday, 2nd Wednesday, etc.

What’s the best way to get this done?

BTW…If there is not an equivalent previous months day that is fine. I can handle that. Also, currently I am counting the number or days (“Mondays”, “Tuesdays”, etc) manually to figure this out. I was just hoping there is a more elegant way to do it.

Answer 1

Here’s what I would do:

static DateTime? GetLastMonthSameNthDayOfWeek(DateTime date)
{
    int nth = (date.Day-1) / 7; // returns 0 if 1st, 1 if 2nd...
    var prevMonthDay = date.AddMonths(-1);

    // find the first date of month having the same day of week
    var d = new DateTime(prevMonthDay.Year, prevMonthDay.Month, 1);
    while(d.Day <= 7)
    {
        if (d.DayOfWeek == date.DayOfWeek)
            break;
        d = d.AddDays(1);
    }
    // go to nth day of week
    d = d.AddDays(7 * nth);
    // if we have passed the current month, there's no nth day of week
    if (d.Month != prevMonthDay.Month)
        return null;
    return d;
}

Usage example:

// 3rd wednesday of August 2012
var a = new DateTime(2012, 8, 15);
var aPrev = GetLastMonthSameNthDayOfWeek(a);
// aPrev = July 18th 2012 (i.e. the 3rd wednesday of July 2012)

// 5th wednesday of August 2012
var b = new DateTime(2012, 8, 15);
var bPrev = GetLastMonthSameNthDayOfWeek(b);
// bPrev = null, because there's no 5th wednesday of July 2012

N.B. :

getting the ordinal position of the day of week inside a month is really easy:

int nth = ((date.Day-1) / 7) + 1; // 1 -> 1st, 2 -> 2nd, 3 -> 3rd ...