Timezone in signatures in (Dutch) nl.wiki''pedia'' is CET/CEST, since the start.
Timezone in nl.wiki''books'' is UTC. Please configure signature timezone in
nl.wikibooks to the same settings as nl.wikipedia, so CET=UTC+1 in winter, and
CEST=UTC+2 in summer.
This had been discussed in the nl.wikibooks village pump equivalent:
[[b:nl:Wikibooks:Lerarenkamer#Tijdzone in ondertekening|Lerarenkamer]] (Dutch),
there was no opposition against it.