Get Day of the Week from Date

This code snippet determines the day of the week associated with a date. The class method "test" takes a date as a string in "mm/dd/yyyy" format, and returns an integer corresponding to a day of the week:

Class cartertiernan.getDayfromDate Extends %RegisteredObject
{
    classmethod test(date) as %Integer {
        //Set date = $ZDATE(date) //  Looks like: mm/dd/yyyy
     
        Set monthList = $LISTBUILD(0,3,3,6,1,4,6,2,5,0,3,5) // (Jan,Feb,Mar,Apr,...)
        Set centuryList = $LISTBUILD(6,4,2,0) // first two digits divisiable by 4, then subsequent centuries. EX (2000, 2100, 2200, 2300)
        Set dayList = $LISTBUILD("Sunday","Monday","Tuesday","Wednesday","Thursday","Friday","Saturday") // Index goes from 0-6
         
        Set day = $PIECE(date,"/",2) // get the day 
        Set monthVal = $LIST(monthList,($PIECE( date,"/",1 ))) // get the month value
        Set first2DigsYear = $PIECE( date,"/",3 ) \ 100 // get the last 2 digits of the year
        Set last2DigsYear = $PIECE( date,"/",3 ) # 100 // get the first 2 digits of the year
         
        // Used for DEBUG perpouses
        /*write !,"day: ",day
        write !,"Month: ",monthVal
        write !,"last2: ",last2DigsYear
        write !,"first2: ",first2DigsYear
        write !,"cen Val: ",$LIST(centuryList,(first2DigsYear # 4) + 1),!!*/
         
        // Look here for formula explination (its the "Basic method for mental calculation")
        // http://en.wikipedia.org/wiki/Determination_of_the_day_of_the_week
        Set dayOfWeekVal = ( day + monthVal + last2DigsYear + (last2DigsYear\4) + $LIST(centuryList,(first2DigsYear # 4) + 1 ) ) # 7
     
        Quit dayOfWeekVal
    }
}

Here's a link to the code on GitHub

(originally posted to CODE by Carter Tiernan, 6/18/14)