Using a string for initialViewDate does not work

### I'm Submitting a ...

```
[ x ] Bug report
[ ] Feature request
[ ] Support request
```

### Steps to Reproduce
Use the Datetime component and provide an initialViewDate string in DateTime format:
e.g. 2021-02-15T13:29:00Z

### Expected Results

When the picker is used it shows the correct Date and time

### Actual Results

The current date is shown and 12:00 is shown for the time

### Minimal Reproduction of the Problem
https://codepen.io/jasonleenaylor/pen/WNoWbze

### Other Information (e.g. stacktraces, related issues, suggestions how to fix)
Setting initialViewDate to a Date constructed with the string works properly.

Open the linked CodePen reproduction and start at the Datetime component's handling of the initialViewDate prop. Compare the behavior when the value is a Date with the reported DateTime-format string. Done means the picker shows the supplied date and time when initialViewDate is provided as a string, while preserving the existing Date behavior.

Written by the indexing model from the issue text.