No you can't, unless you write your own version of the module (edit: which I think is a bad idea BTW). DateTime sets default values in two different places. First in new, a validator is used, with the entries:

            hour => {
                type    => t('Hour'),
                default => 0,
            },
            minute => {
                type    => t('Minute'),
                default => 0,
            },
            second => {
                type    => t('Second'),
                default => 0,
            },
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Then new calls _new, which sets

    # If this method is called from somewhere other than new(), then s
+ome of
    # these defaults may not get applied.
    ...
    $p{hour}       = 0                          unless exists $p{hour}
+;
    $p{minute}     = 0                          unless exists $p{minut
+e};
    $p{second}     = 0                          unless exists $p{secon
+d};
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After that there's no difference between defining those values to 0, or not defining them from the caller.