Adding Days to Julian date

Hi,

I'm working on writing a query to figure our the rent period for Invoice. I have an invoice date and billing frequency, I will use Invoice date as rent period start date and based on the billing frequency will add number of days to rent period start date i,e invoice date and derive an rent period end date.

Invoice date/Rent Period Start Date: 12/01/2014 (Converted From Julian Date: 114335)

Rent Period End Date: I added 182 days to Invoice days, Julian Value after addition is 114517..(That by no means can be converted to any date)

How to add days to julian date is big issue i'm facing now, any help is appreciated..