days remaining in sf formula

1. click on setup
2. go to build menu
3. expand appropriate sfobject 
4. click on fileds

or

1. expand force.com quick access menu
2. click on view fields

Goto custom fileds & relationships

click on new

select datatype as 'formula' and click on next

provide filed lable name upon field label name filed field name will auto populate

then select Formula Return Type  click on next

enter formula in formula editor window

 ex: EndDate  - today()

click on next and provide visible and readonly access click n next

select pagelayout and save