Question: Is there a extjs based datetimepicker?

I want user can select date and time together.

Answer: 
Unfortunately there ***is not*** a DateTimePicker out of the box, but there are two separate components named `TimeField` and `DateField` that can be used in conjunction to get the functionality you desire.

If you'd rather use Saki's `DateTime` user extension, take a look at [DateTime Field](http://www.sencha.com/forum/showthread.php?22661-new-version-DateTime-Field). The class comments describe DateTime as "[a] combination of DateField and TimeField", so nothing too crazy is going on if you'd like to create your own unique version.
There's also [this one](http://www.sencha.com/forum/showthread.php?137242-Ext.ux.DateTimeField-DateTimePicker-for-ext4-also-DateTimeMenu-TimePickerField), works in ExtJS 4.1, but uses number spinners for selecting time, can be reworked relatively easily to use the built-in time picker.