20.1.1 ab = since, from (with Dative)

  timely relation
  Er ist ab dem 15. Januar nicht mehr im Büro.
= From January 5, he will not be in the office anymore.
  Ab diesem Zeitpunkt machte jeder, was er wollte.
= Since then every one did whatever he felt like.
  Ab dem Bahnhof weisen Schilder den Weg.
= From the station signs mark the way.

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