Great :-) I also came across this interface once and thought it would be nice to have for ObsPy!

Any reason for not using the default string representation of the `UTCDateTime` objects? It by chance corresponds to the default JSON date representation of Javascript and is also way more readable.

```javascript
var t = new Date("2014-03-05T21:59:03.039612Z")
// Prints "2014-03-05T21:59:03.039Z"
console.log(t.toJSON())
```