Ensure proper datedepublish format

When saving content, we must make sure that the user hasn't entered an incorrect value for datedepublish (e.g. empty time-value). Otherwise bolt explodes!
 
![img_1908](https://cloud.githubusercontent.com/assets/1737019/2888156/4626df48-d506-11e3-9c8c-4cb95499143b.png)