Description of problem:

Let's have a vm that is retired, navigate to the vm's "Set/Remove retirement date for Virtual Machine" page. 

When the "Retirement Date and Time" field is clicked, the date is removed, a date widget is displayed, but the save button is not activated.

I would expect 2 following behaviours:
1. When user selects "Retirement Date and Time", the date is cleared and the Save button is activated, so that when the date widget is closed, changes can be saved.

2. When user selects "Retirement Date and Time", the date remains the same, and when clearing the field manually or by clicking the thrash bin, the Save button is activated, so changes can be saved. 

The first case feels more natural.

Version-Release number of selected component (if applicable):
CFME 5.9.0.16.20180109204148_7ac9852 

How reproducible:
100%

Steps to Reproduce:
1. Navigat to Set/Remove retirement date for Virtual Machine of a retired VM
2. Select Retirement Date and Time field and close it by clicking anywhere outside the widget

Actual results:
Save button is not activated, but Date has been changed (removed).

Expected results:
Save button IS activated, so that changes can be saved

Additional info:
For exact steps you may refer to test cfme.tests.cloud_infra_common/test_retirement/test_resume_retired_instance