Date difference with multiple criteria

Step 1 – Get milestone dates per person

You want the first occurrence of each role.

Volunteer → Spanish Training

First Volunteer Orientation =
CALCULATE (
    MIN ( Assignments[Service From Date] ),
    Assignments[Service Assignment] = "Volunteer Orientation"
)

First Spanish Training =
CALCULATE (
    MIN ( Assignments[Service From Date] ),
    Assignments[Service Assignment] = "Interpreter: Spanish Training"
)

First Spanish Remote

First Spanish Remote =
CALCULATE (
    MIN ( Assignments[Service From Date] ),
    Assignments[Service Assignment] = "Interpreter: Spanish Remote"
)

These measures will return the correct first date per Volunteer, as long as Volunteer is on the visual (or in filter context).

Step 2 – Date differences

Orientation → Training

Days: Orientation to Training =
VAR StartDate = [First Volunteer Orientation]
VAR EndDate   = [First Spanish Training]
RETURN
IF (
    NOT ISBLANK ( StartDate ) && NOT ISBLANK ( EndDate ),
    DATEDIFF ( StartDate, EndDate, DAY )
)

Training → First Remote

Days: Training to First Remote =
VAR StartDate = [First Spanish Training]
VAR EndDate   = [First Spanish Remote]
RETURN
IF (
    NOT ISBLANK ( StartDate ) && NOT ISBLANK ( EndDate ),
    DATEDIFF ( StartDate, EndDate, DAY )
)