CALCULATE Error in Measure

[WooPig]

This is probably pretty basic stuff, but I'm new to DAX and it's got me stumped.

This measure works for finding our most recent record:
MaxCompDt:=CALCULATE(LASTDATE(Completed[Completion Date]),all(Completed))

And this measure works for finding the first day of whatever month that record is from:
1stDOM:=CALCULATE(STARTOFMONTH(LASTDATE(Completed[Completion Date])),all(Completed))

But if I try to use the first measure inside the second measure to replace the LASTDATE part:
1stDOM:=CALCULATE(STARTOFMONTH([MaxCompDt]),all(Completed))

Then I get the "CALCULATE has been used in a true/false expression" error. The two working measures are being used for a DATESBETWEEN calculation, which is working. So really everything works. This is just more a curiosity so that I can learn something.

So I have two questions.

1. Why does this trigger that error?
2. What is best practice here? Should I use the first measure inside the second measure, or is that not a good idea?

 

[scottsen]

Typically that sorta thing "just works", but as you have seen... sometimes that doesn't work out. I would be curious to see if replacing LASTDATE with MAX makes it work? There are some subtle differences between the two which might help your luck.

Note also, I wouldn't expect the all(completed) on your 1stDOM measure, since MaxCompDt took care of that?

 

[WooPig]

Typically that sorta thing "just works", but as you have seen... sometimes that doesn't work out. I would be curious to see if replacing LASTDATE with MAX makes it work? There are some subtle differences between the two which might help your luck.

Note also, I wouldn't expect the all(completed) on your 1stDOM measure, since MaxCompDt took care of that?

I did try using MAX instead of LASTDATE, but nothing changed. I had another situation last week where using MAX had worked when LASTDATE didn't, but no luck here.