How to use dow? and for what can it be useful?
On the first question... it is rather simple:
Click to choose the date... use context menu to add or subtract months or years.
Click the calendar day, to get:
the week day for the selected date...
and the number of days so far (or to come) between today and your date.
On the second question: it is nearly as simple... dow is useful for nothing!
... Hmmm... may be you can use it to add or subtract days!
Select the date
Use context menus to quickly go to far dates