First of all you have to make sure that you understand how server-side programming and client-side programming works. ASP.NET is server-side and javascripting is client-side. The topic has been adressed a lot of times in this forum.
Enough said about that... If the path you are writing are correct the file will be included. However, you should - as you mention yourself - write the path relative to the document in which you put the reference. Say you have the script in a subfolder called
mySubFolder relative to the document which are using the script, then the path would be...
mySubFolder\datetimepicker.js
I am betting that your problem is rather a misunderstanding about server/client-side concepts.
Hope it helps, Jacob.