Should Schema.org dateModified have some default value if not available?

Question

For Schema.org Article I get an error in https://developers.google.com/structured-data/testing-tool/ for dateModified:

The dateModified fiel

Answer 1

<span itemprop='dateModified'>09/05/2017t02:25</span> or
<meta content='09/05/2017t02:25' itemprop='dateModified'/>
<div itemprop='dateModified'>29.05.2017T02:30</div>

Answer 2

The SDTT is not a general Schema.org validator. It primarily checks the Schema.org-based structured data for Google’s own search result features. If Google’s tool reports an error, it doesn’t necessarily mean that your markup is wrong, it often only means that you won’t get a certain Google search result feature.

The vocabulary Schema.org doesn’t require any property. It’s perfectly fine not to provide the dateModified property, and it’s the right thing to do if your publication wasn’t modified yet.

(FWIW, the documentation for Google’s Top Stories with AMP feature says, for publications that were not modified, to omit the property or to set it to the same value as datePublished.)