tracker issue : CF-4201968

Problem Description:
When editing a scheduled task, if validation fails and there's a date in the end date field, it turns into a timestamp when the page reloads which doesn't save.

Steps to Reproduce:
Create scheduled task with a start and end date
Edit the task and cause a validation failure.  Not just any validation failure will do it.  it seems that the validation failure must be related to the dates on the task.  For instance, I set a start date of 04/12/2018 and an end date of 04/13/2018 and set the frequency to "crontime" and pasted "0 0 12 * * ?" in the cron box.  This generated the following validation error:

> An error occurred scheduling the task.
> Based on configured schedule, the given trigger 'SERVERSCHEDULETASK#$%^GROUP.trigger_TASK NAME' will never fire.

That error message is a bit bad since it uses the internal struct key used to store the task which is not human readable at all.  
When the form reloaded, the end date was replaced with this value: "{ts '2018-04-13 00:00:00'}"
This caused subsequent saves to fail with the validation message: 
> You need to enter a valid End Date.

Actual Result:
End date is converted to timestamp
Error message was bad

Expected Result:
End date should not be converted to timestamp
Error message should not be bad

Any Workarounds:
None

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