Date tables and subsequent columns

[mwen90]

Hi, 

I have a table with a start date and finish date.

I then have a date table related to this table. It has a column with a day of the year, so a start date could be day number 450.

Each of these dates are categorised by a category and level. I want to calculate the first date (day number 300) minus the last date of the category (day number 350). 

I have no idea how to do this. How can I relate the day number to the start date?

ive tried relationships and lookupvalue with no luck,

Thanks so much for the help!

[amitchandak]

@mwen90 ,Can you share sample data and sample output in table format? Or a sample pbix after removing sensitive data.

[mwen90]

Thanks @amitchandak  please see here, I think I summarized the issue much better! https://community.powerbi.com/t5/Desktop/Calculate-Difference-in-Column/m-p/2047145#M766090 

Apologies for the confusion, this issue has got me! 

Thanks again for the help