Hi @funkyplanet , that property looks valid, I'd just double check that there aren't any spaces or extra characters hidden there. The property should be lastmodifieddatewhich it looks like you're doing already. That said, I won't have much insight on the Zapier side of things, so I do believe that Zapier would be the best place for support on this. The only other thing I can think of is that Zapier doesn't allow zaps based on the last modified date property, as this would send off a zap every time a contact did anything.
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