Skip redundant units in time-axis

Now ticks won't be intersect on small screen/
Before:
![image](https://f.cloud.github.com/assets/812240/1887979/d5d13a4a-7a06-11e3-9ecc-5d564718abf7.png)

After:
![image](https://f.cloud.github.com/assets/812240/1887991/f7be5110-7a06-11e3-8573-9a6aef10a346.png)