Im not sure I understand. This is what I THINK you mean.
Lets say the value is 10. The start date is the 15th March, End date is 20th March (5 days). Spreading 10 over 5 days gives you 2 per day?
If that is what you mean then assuming that:
A1 = Start Date
B1 = Completion Date
C1 = 'Value' that you want spread evenly across the range
D1 = C1/(DATEDIF(A1,B1,"d"))
Make sure to format D1 as a number not a date
[Edit] Meant D1 not C1 in the last sentence