What steps I need to follow to achieve this?
Right, Please see below;
To disable time picker: Click edit -> Edit panels -> push the x next to the timepicker
To add prompts, which I believe you mean 2 input fields, you can use the form elements and set tokens, i.e. $fromdate$
To add constraints, you could use a number field, and set a maximum number of 15, or alternatively, use a drop-down with -1d, -2d, -3d etc.
Hope this is what you're looking for, if not leave a comment and I'll get back to you