FIX: use short date format for timestamp

Before:

![screen shot 2014-08-27 at 11 43 44](https://cloud.githubusercontent.com/assets/5732281/4055949/5ffc8612-2db1-11e4-8624-deca890e1b7b.png)

After:

- for current year:

![screen shot 2014-08-27 at 11 29 32](https://cloud.githubusercontent.com/assets/5732281/4055952/7232cb70-2db1-11e4-9a0a-4fc9048057f0.png)

- other than current year:

![screen shot 2014-08-27 at 11 37 29](https://cloud.githubusercontent.com/assets/5732281/4055956/7caf9218-2db1-11e4-94cc-ab3312446ccc.png)

cc @coding-horror