I believe I have found an error in the CAA DTUtils GetDayOfWeek function. According to the documentation, it should return 0-6 for days of the week, Sunday-Saturday. Today, I'm getting a return value of 7, and an error of DTU_ERROR_UNKNOWN.
I have library version 3.5.12.0. Anyone else have this problem?

Hi,
Yes, it is an known bug.
The entry in our developer database is:
CDS-47688: Library: CAA DTUtility GetDayOfWeek returns wrong weekday for leap day

Regards Thomas

@ThK you seem to have access to this developer database. Can you explain why this kind of information is not publicly available? I used the CAA library because I made the assumption that a date and time utility would have been tested against dates and times, leap years being one of the most obvious tests. Knowing that it doesn't work, I don't see the use of it--I might as well write my own function.

For anyone who needs to replace it, here's a function that works

FUNCTION DATE_TO_DAY : INT(0..6)
VAR_INPUT
    DateIn:                 DATE;
END_VAR
VAR CONSTANT
    SECONDS_PER_DAY:        DINT:= 24*60*60;    //number of seconds in a day
    EPOCH_STARTDAY:         DINT:= 4;           //DATE variable is stored as seconds since Jan 1, 1970, which was a Thursday 
END_VAR

(*====================================================================================
DATE_TO_DAY
Calculate day of week from date 
Sunday=0, Saturday=6
======================================================================================*)

DATE_TO_DAY := DINT_TO_INT(((DATE_TO_DINT(DateIn) / SECONDS_PER_DAY) + EPOCH_STARTDAY) MOD 7);