Can't calculate days with two formulas in 1 column

[Beomere]

I have 2 columns amongst many:

Created Date and Completed Date

All cells under Created Date have an entry, but not all under Completed Date do. I'd like to create a Custom Column with a formula that tells me the number of days (in a 7 day week, not a 5 day working week) between Completed and Created Date; and where no Completed Date exists, between Today's Date (which is dynamic) and the Created Date. Can anyone please help as I can't understand IF formulas. 

[Beomere]

got it now, thank you!

[amitchandak · Accepted solution]

@Beomere ,

A new column

Total days =

if(not(isblank([Completed Date])), datediff( [Created Date] ,[Completed Date], day), blank())

or

Business Days =

if(not(isblank([Completed Date])), COUNTROWS(FILTER(ADDCOLUMNS(CALENDAR(Table[Created Date],Table[Completed Date]),"WorkDay", if(WEEKDAY([Date],2) <6,1,0)),[WorkDay] =1)), blank())