Serialize datetime objects into the module config

[dagardner-nv]

Description

• Fixes an issue where the --start_time flag was being ignored
• Prior to Update cast_from_pyobject to throw on unsupported types rather than returning null MRC#451 the datetime objects being stored in the config were being serialized to json null, after that change an exception was raised.

Closes #1590

By Submitting this PR I confirm:

• I am familiar with the Contributing Guidelines.
• When the PR is ready for review, new or existing tests cover these changes.
• When the PR is ready for review, the documentation is up to date with these changes.

[dagardner-nv]

/merge