Display day number in textbox on date picker value

Guys,

 

I'm trying to write a little function that when my datetime picker has a date selected by the user it shows what day number in the year it is in textbox1.

 

So for instance in my datetime picker changed event I gues say someone selcts 1-1-2015 technically it would display '1' in textbox1, however if they then slected 1-2-2015 it would display '32' in textbox1.

 

And obviously if they selected 31-12-2015 it would be '365' in textbox1 one.

 

Can such a thing be done? if and obviously the code to count the days I would place in the datatime picker selection changed code..