The validity period of a visa and the granted duration of stay are different things. You cannot necessarily stay in Denmark for the entire period of validity.
A visa will always be issued with a validity that exceeds the number of days that the applicant has been granted permission to stay in Schengen, in order to allow the traveller some flexibility.
For example: If you have applied for a 10 days visa, the Embassy will give you 15 days of grace, in total 25 days during which you can stay 10 days. I.e. the allowed period of stay will be 10 days as you applied for.
A visa will not be granted for stays exceeding 90 days in any 180-day period. The counting of the 90 days includes stays in all Schengen countries. The 90 days may be spread across one or more visits, depending on the number of entries you have been granted.
Entry and exit is not allowed outside the validity period, and the length of the visit cannot exceed the granted number of days. Both the validity period during which the traveller can enter Schengen and the length (days) of the granted stay is printed on the visa.
If you remain in Denmark after your visa expires or if you attempt to use your visa stay to obtain permanent or long-term residency in Denmark, you can be given a penalty period of three or five years where a visa can not be obtained.
Schengen visa issued for stay of 90 days within the period 01-07-2013 to 13-10-2013. The length of the visit cannot exceed the granted number of days, and entry and exit is not allowed outside the validity period.