Remove error when submitting the publication form with the date picker element.

Before fix:
![before 2](https://cloud.githubusercontent.com/assets/1222368/2789283/cfd1f65c-cbaa-11e3-8ead-33b05a2c4c73.png)

After fix:
![before 1](https://cloud.githubusercontent.com/assets/1222368/2789282/ca9d9506-cbaa-11e3-990b-0e2c06470e96.png)