Measure Difference When Value = Value - 1

[JRoot213]

I have a measure, CRT, that is being displayed over an interval of every 30 minutes time. to make things easier I've attached an interval ID number that represents every 30 minutes of the day. 

I'm trying to measure the difference between the measure 30 minutes ago compared to the current time. I am coming up short. Currently I have the below attempt in DAX

CRT Difference =

VAR PrevCRT =

CALCULATE(IntraCRT[CRT],

FILTER(DAXIntervalID,

    DAXIntervalID[Value] = DAXIntervalID[Value] -1)

)

VAR CurrentCRT = IntraCRT[CRT]

RETURN

    PrevCRT - CurrentCRT

The outcome I receive for PrevCRT is blanks vs the CRT from 30 minutes prior. 

[JRoot213]

Thank you so much for the quick reply. 

Unfortunately that's giving me the same result.

[wdx223_Daniel]

CRT Difference =

VAR PrevCRT =

CALCULATE(IntraCRT[CRT],

FILTER(ALL(DAXIntervalID[Value]),

    DAXIntervalID[Value] = MAX(DAXIntervalID[Value]) -1)

)

VAR CurrentCRT = IntraCRT[CRT]

RETURN

    PrevCRT - CurrentCRT

[wdx223_Daniel]

CRT Difference =

VAR PrevCRT =

CALCULATE(IntraCRT[CRT],

FILTER(ALL(DAXIntervalID[Value]),

    DAXIntervalID[Value] = DAXIntervalID[Value] -1)

)

VAR CurrentCRT = IntraCRT[CRT]

RETURN

    PrevCRT - CurrentCRT