Bugfix: datetime.date's in validate_date()

[dpb-bah]

High level description:
validate_date() was only checking against datetime.datetime's and not datetime.date's

Technical details:
(see high level description)

Link to JIRA Ticket:
(N/A)

The following are ALL required for the PR to be merged:

• Backend review completed
• Matview impact assessment completed (N/A)
• Frontend impact assessment completed (N/A)
• Data validation completed (N/A)
• API Performance evaluation completed and present (N/A)