Bug 11244: (follow-up) Fix $dateonly flag

Bug 11244: (follow-up) Fix $dateonly flag

At the moment, $dateonly is set to true when $1 is defined. However,
since the regex capture group only includes the time, this flag will
only be set when there is a value that includes a time.

In effect, this means that timestamps are reduced to dates only,
while dates have 00-00-0000 added to them.

This patch keeps the logic but reverses the values, so that $dateonly
will default to true unless $1 is defined.

Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>
Signed-off-by: Jonathan Druart <jonathan.druart@biblibre.com>
Signed-off-by: Tomas Cohen Arazi <tomascohen@gmail.com>