Hi Jeremy,
JWTs expect iat (issued at) and exp (expiration) as integers in Unix time format. This format counts seconds since 1970-01-01T00:00:00Z and is usually represented as a Long or Integer, not as a DateTime object. APIs expecting JWTs will look for integer values for these fields.
you can change the type of iat and exp in long or int in entity use microflow to calculate Generate Unix Timestamp in seconds.
Thanks,
Arun