consider the timezone when displaying the day calendaric value.

#2407 
![2407](https://f.cloud.github.com/assets/1222368/642970/3b7a29ee-d36b-11e2-86d8-74c58922e4cc.png)

The value of the field has been saved as UTC timezone and when the field was rendered, the date module adjust him to the site timezone. In the os_basetheme_preprocess_node(), the value of the field was converted to timestamp using the strtotime function but the function didn't consider the timezone. What happens is that the rendered output and the value in the value has 4 hours difference and then the user saw 16th on the left and 15th on the right.

In this PR i consider the timezone and the result are reflected in the screen shot.