"svnadmin dump" "canonicalizes" each svn:date revprop while dumping, in the function write_revision_record().
This seems to have been done in r842390 in order to upgrade from pre-0.14 repository format to the new timestamp format introduced in 0.14 – see issue
SVN-614 "DAV:creationdate needs to be an ISO8601 date". svn_time_from_cstring() reads either new or old format, and then svn_time_to_cstring() writes the new format.
However, this does not only convert old to new format, but could also make textual changes to the string if the revprop value is not already canonical. Dump should carefully output exactly what is in the repository and not gratuitously change it. In retrospect, such a transformation should have been done during "svnadmin load" instead of in "dump".
While "svnadmin dump" makes this transformation, "svnrdump dump" and "svndumpfilter" do not. This could lead to unintended differences in dump output depending on which tool is used. (I made some progress in unifying the output logic for those three dump producers a couple of years ago, but I left this part alone because I did not know what to do with it.)