DATEDIFF between 2 dates in different tables

Anonymous 

Please use 

Minutes =
SUMX (
    CROSSJOIN (
        SUMMARIZE ( 'Link Table', dim_date[Date], dim_date[Year] ),
        SUMMARIZE (
            'Link Table',
            'Link Table'[dim_professional_carer_key],
            'Link Table'[dim_service_points_key]
        )
    ),
    CALCULATE (
        DATEDIFF (
            SELECTEDVALUE ( 'fact_service_point_sessions'[START_DTTM] ),
            SELECTEDVALUE ( 'fact day therapy theatre events'[Earliest Date] ),
            MINUTE
        ),
        'fact day therapy theatre events'[First Patient] = "First"
    )
)