Hi,

It doesn't look like DateTime can handle %a returning only 2 chars. -- No Idea why :-)

That's not it at all. Please see the documentation for DateTime::Locale. You can look up the data used by DateTime::Locale for each locale code, for example, for the one you used, de, at https://metacpan.org/pod/distribution/DateTime-Locale/lib/DateTime/Locale/de.pod, which contains:

Abbreviated (format)

Mo.
Di.
Mi.
Do.
Fr.
Sa.
So.

...

Abbreviated (stand-alone)

Mo
Di
Mi
Do
Fr
Sa
So
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So you should use DateTime::format_cldr() with format ccc to print "Abbreviated (stand-alone)", as shown in the docs:

$ perl -Mstrict -MDateTime -wlE 'say join", ", map { DateTime->new( lo
+cale => "de", year => 1970, month => 1, day => 4 + $_ )->format_cldr(
+"ccc") } 0 .. 6'

So, Mo, Di, Mi, Do, Fr, Sa
[download]