Added date checking to fireplace (bug 1044230)

> I feel out of my league to make a call here. If you can advise me how best to handle this I'll implement.

well, what is the issue? if it's just backwards compatibility for flexbox, just do the following:

```css
#error-overlay > .error-flex {
    -moz-box-align: center;
    align-items: center;
    display: -moz-flex;
    display: flex;
    height: 100%;
    -moz-box-pack: center;
    justify-content: center;
    width: 100%;
}
```

we have [flexbox mixins](https://github.com/mozilla/fireplace/blob/4071259/src/media/css/lib.styl#L298) but I wouldn't recommend them because they're overly verbose.

I'd also take a look at http://philipwalton.github.io/solved-by-flexbox/demos/vertical-centering/ and inspect the styles to see which vendor prefixes you might need. it's a great resource!