Bug 25700: (follow-up) Minor corrections to epoch dates

Bug 25700: (follow-up) Minor corrections to epoch dates

It looks like we forgot to account for the year change when grabbing
epoch dates for some of the later KohaCon's. This patch fixes those
minor mistakes.

Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>
Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>
Signed-off-by: David Nind <david@davidnind.com>
Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>