I am using the following code for publish date and modified date in schema markup of a blogger site.

    "datePublished": "<data:post.date.iso8601/>",
    "dateModified": "<data:post.date.iso8601/>",

but when I check it on Schema Markup validator is shows "<! --Can't find substitution for tag [post.date.iso8601]--> " for both value, please help me to fix it.