Day of week for a specific date value

AFL Programming
1

Hello Guys,
How to check day of week for a specific date value?
For example, I need something like: _DayOfWeek("2026-01-08") that would return 4 (Thursday).
Any suggestions?

2

You can use the Lookup() function for that.

3

In the future, not in the past. Specifically, to check what day of the week a given date will fall on.

4

This is a very famous problem and many have written algorithms for it.

From wiki
see Tomohiko Sakamoto's Algorithm

dayofweek(y, m, d)	/* 1 <= m <= 12, y > 1752 (in the U.K.) */
{
    static int t[] = {0, 3, 2, 5, 3, 0, 5, 1, 4, 6, 2, 4};
    if ( m < 3 )
    {
        y -= 1;
    }
    return (y + y/4 - y/400 + y/100 + t[m-1] + d) % 7;
}

Easy to rewrite in AFL
Determination of the day of the week - Wikipedia

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5

Calculate difference between given day and today in days. Calculate modulus 7 (reminder from division) and knowing what day is today you would now what day given date is.

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6

Along the line of what Tomasz wrote, here is what I've used in several apps. In this way DateTimeDiff() is doing the details of leap year stuff -

	//  futuredtstr is input
	futuredt		= StrToDateTime( futuredtstr );
	//  This form allows negative offsets from current last date
	futurewd		= ( LastValue( DayOfWeek() ) + ( DateTimeDiff( futuredt, LastValue( DateTime() ) ) / (60 * 60 * 24) ) % 7 ) % 7;
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7

My code variation

_datestring="2026-01-08";
_datetime=StrToDateTime(_datestring);
//_datetime=DateTimeAdd(_datetime, 1, inDaily);//from date plus x days
_dayNum = DateTimeConvert(0, _datetime);
_weekDay = (_dayNum +1)% 7;
 
 "_weekDay: "+_weekDay;
8

You oversimplified and your solution is incorrect. Try it with 2026-02-08 and you’ll see that it produces the wrong DOW.

9

Yes, you are right.

10 

//https://www.geeksforgeeks.org/dsa/find-day-of-the-week-for-a-given-date/
// Function to calculate the day of the week using the formula-based approach
function f_dayOfWeek( _datetime )
{
    _datetime_DateOnly = DateTimeToStr( _datetime, 4 ); //iso date only YYYY-MM-DD
    y = StrToNum( StrMid( _datetime_DateOnly, 0, 4 ) );
    m = StrToNum( StrMid( _datetime_DateOnly, 5, 2 ) );
    d = StrToNum( StrMid( _datetime_DateOnly, 8, 2 ) );

    // Predefined month codes for each month
    monthCode[0] = 6;
    monthCode[1] = 2;
    monthCode[2] = 2;
    monthCode[3] = 5;

    monthCode[4] = 0;
    monthCode[5] = 3;
    monthCode[6] = 5;
    monthCode[7] = 1;

    monthCode[8] = 4;
    monthCode[9] = 6;
    monthCode[10] = 2;
    monthCode[11] = 4;

    // Adjust year for January and February
    if( m < 3 )
    {
        y -= 1;  // If month is January or February, treat them as part of the previous year
    }

    // Calculate the year code
    yearCode = ( y % 100 ) + ( y % 100 ) / 4;

    // Adjust year code for the century
    yearCode = ( yearCode + ( y / 100 ) / 4 + 5 * ( y / 100 ) ) % 7;

    // Calculate the day of the week and return the value as an integer
    return ( d + monthCode[m - 1] + yearCode ) % 7;
}

//_datetime = DateTimeAdd( LastValue( DateTime() ), 1, inDaily );
_datetime = StrToDateTime( "2026-02-08" );

result = floor( f_dayOfWeek( _datetime ) );
"f_dayOfWeek: " + result;
11 
futuredtstr = "2026-12-12";
futurewd = DateTimeFormat( "%w", StrToDateTime( futuredtstr ) );
// returns string

// also subject to upper limit of StrToDateTime() depending on AB version 6/7

12

That is Zeller's algo, but I think Sakamoto(the one I posted) is the more modernly used approach.
The lookup table is a bit more efficient.

In my plugin journey, some optimizations came better with switch-case vs longer if-else(for runtime strings this was better), and the learning continues.

13

Sorry, I forgot that I already implemented that functionality in DateTimeConvert function:

function StrToWeekday( string )
{
  return DateTimeConvert( 9, StrToDateTime( string ) );
}

printf("%g\n", StrToWeekday("2026-01-09") );

In Release Notes it is documented:

CHANGES FOR VERSION 6.16.0 (as compared to 6.15.0)
AFL: DateTimeConvert supports now more formats, format = 6 - day, 7 - month, 8 - year, 9 - day of week, 10 - day of year, 11 - quarter (1..4)

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