Measure to find oldest Project by subtracting oldest project and today

[Datagulf]

I have a table that looks like the one below. Each entry is a single project. 

Client Name	Project Entry Date	Project Completed Date	Ageing in Days	Project Status
Client A	22/01/2019	01/04/2019	69 days	Completed
Client B	13/07/2020			Ongoing
Client C	17/06/2021	19/092021	103 days	Completed
Client D	21/09/2021	29/09/2021	8 days	Completed
Client E	23/10/2021	06/01/2022	74 days	Completed
CLient F	27/01/2022			Ongoing

So I want to get the number of days of an ongoing project that has stayed the most. That project would probably be the one for Client B since it does not have a completed date and is the earliest. 

I am thinking Datediff would work with and then get a value of the highest using RankX , DENSE. I want to display that number of days on a card. Any assistance is highly appreciated. 

[Adescrit]

How about this:

Oldest =
VAR __Days =
    MAXX (
        'Table',
        IF (
            ISBLANK ( 'Table'[Completed Date] ),
            DATEDIFF ( 'Table'[Entry Date], TODAY (), DAY ),
            DATEDIFF ( 'Table'[Entry Date], 'Table'[Completed Date], DAY )
        )
    )
RETURN
    __Days

[Datagulf]

Thanks for this. However, it is somewhat not correct from my side's explanation. I have been able to extract a date with the oldest date of an ongoing project. The formula used is as follows. 

Oldest project =
CALCULATE (
    MIN ( 'table'[entry_date] ),
    FILTER (
        'table',
        'table'[Status] <> "completed"
            && 'table'[Status] <> "removed"
            && 'table'[Status] <> "cancelled"
    )
)

how can I subtract this date from today's Date using Today()? @Adescrit 

[Adescrit · Accepted solution]

Oldest project = 
VAR __Date = CALCULATE (
    MIN ( 'Table'[Entry Date] ),
    FILTER (
        'table',
        'table'[Status] <> "completed"
            && 'table'[Status] <> "removed"
            && 'table'[Status] <> "cancelled"
    )
)
RETURN
    
    DATEDIFF( __Date, TODAY(), DAY)

[Datagulf]

In my answer, it returns a blank. Not sure why. Thanks, though

[Datagulf]

Finally it works