Hi,
as WhatsApp uses a closed protcol, and there is no official API all libraries that exist seem to be more or less hacks using an unofficial API and "emulating" official WhatsApp clients.
If absolutely needed, i would also go the way Matthew described, using the Alarm Notifiers. Have in mind that the API could change anytime, and your solution might not be working anymore from one day to the other.
If possible, i would opt in for a more open solution, with n official API like XMPP/Jabber or Slack.
Best regards
Marco