L2CPBG

mailbox.org is sending (at least) a wrong getlastmodified date/time format.

Here's the response body of my test contact:

<?xml version="1.0" encoding="UTF-8"?>
<D:multistatus xmlns:D="DAV:" xmlns:CARD="urn:ietf:params:xml:ns:carddav" xmlns:MM="http://me.com/_namespace/">
  <D:response>
    <D:href>/carddav/6/5759d948-2c59-4402-b86e-154b6b9a0fbb.vcf</D:href>
    <D:propstat>
      <D:prop>
        <D:getetag>2746089-2-1762456395763</D:getetag>
        <D:getlastmodified xmlns:b="urn:uuid:c2f41010-65b3-11d1-a29f-00aa00c14882/" b:dt="dateTime.rfc1123">Thu, 06 Nov 2025 19:13:15 UTC</D:getlastmodified>
        <address-data xmlns="urn:ietf:params:xml:ns:carddav"><![CDATA[BEGIN:VCARD
VERSION:3.0
EMAIL;TYPE=work:test66@mailbox.org
FN:Jörg Ebeling
PRODID:-//Open-Xchange//8.42.63//EN
REV:2025-11-06T19:13:15Z
N:Ebeling;Jörg
TEL;TYPE=cell:0XXX XXX 00 80
UID:5759d948-2c59-4402-b86e-154b6b9a0fbb
END:VCARD]]></address-data>
      </D:prop>
      <D:status>HTTP/1.1 200 OK</D:status>
    </D:propstat>
  </D:response>
</D:multistatus>

Relevant reference: RFC 4918 Section 15.7 - getlastmodified Property which say:

...
Value:   rfc1123-date (defined in Section 3.3.1 of [RFC2616])
...

whereas Section 3.3.1 of [RFC2616] writes:

HTTP applications have historically allowed three different formats
   for the representation of date/time stamps:

      Sun, 06 Nov 1994 08:49:37 GMT  ; RFC 822, updated by RFC 1123
      Sunday, 06-Nov-94 08:49:37 GMT ; RFC 850, obsoleted by RFC 1036
      Sun Nov  6 08:49:37 1994       ; ANSI C's asctime() format

   The first format is preferred as an Internet standard and represents
   a fixed-length subset of that defined by RFC 1123 [8] (an update to
   RFC 822 [9]). The second format is in common use, but is based on the
   obsolete RFC 850 [12] date format and lacks a four-digit year.
   HTTP/1.1 clients and servers that parse the date value MUST accept
   all three formats (for compatibility with HTTP/1.0), though they MUST
   only generate the RFC 1123 format for representing HTTP-date values
   in header fields. See section 19.3 for further information.

5.2.14 of RFC 1123 states:

The syntax for the date is hereby changed to:

            date = 1*2DIGIT month 2*4DIGIT

All mail software SHOULD use 4-digit years in dates, to ease
         the transition to the next century.

         There is a strong trend towards the use of numeric timezone
         indicators, and implementations SHOULD use numeric timezones
         instead of timezone names.  However, all implementations MUST
         accept either notation.  If timezone names are used, they MUST
         be exactly as defined in RFC-822.
....

So we have to support reading of RFC 822 but that clearly states:

...
zone        =  "UT"  / "GMT"                ; Universal Time
                                                 ; North American : UT
                 /  "EST" / "EDT"                ;  Eastern:  - 5/ - 4
                 /  "CST" / "CDT"                ;  Central:  - 6/ - 5
                 /  "MST" / "MDT"                ;  Mountain: - 7/ - 6
                 /  "PST" / "PDT"                ;  Pacific:  - 8/ - 7
                 /  1ALPHA                       ; Military: Z = UT;
                                                 ;  A:-1; (J not used)
                                                 ;  M:-12; N:+1; Y:+12
                 / ( ("+" / "-") 4DIGIT )        ; Local differential
                                                 ;  hours+min. (HHMM)
...

But not "UTC"