Struggling with DATEDIFF

Hi, this is my first post and my first report where I am trying dynamic date/time measures and visuals, not much success so far but almost certainly down to user error.

I am trying to find the difference in days between two dates,

the first is a measure:

LastDate = CALCULATE(MAX('Calendar'[Date].[Date]),FILTER(Table1,Table1[Account Manager]<>""))

which appears to be working as when i put this into a card I get a date formatted correct answer.

the second is a column of date values

I think these are properly formatted?

I have then created a new column called "IsMonth" shown above far right.

The function in this column is:

IsMonth = DATEDIFF(Table1[Input Date],[LastDate],DAY)

But the output seems to be zero all the time, I have tried checking out solutions on the forum but can't see what I am doing wrong here.

Incidentally once I have this working I intend to extend the function to create a 1,0 flag in the column with something like:

IsMonth = IF(DATEDIFF(Table1[Input Date],[LastDate],DAY)<30,1,0)

where 1 indicates the Table1[Input Date] is in the last 30 days and 0 otherwise. And then to use this as a filter in visuals.

Thanks in anticipation

Andy