This MTD or Last MTD on 1st of month

 (

DATEPART( 'day', TODAY() ) = 1

AND DATEADD( 'month', -1, DATETRUNC( 'month', TODAY() ) ) = DATETRUNC( 'month', [Trip Date] )

)

OR

(

DATEPART( 'day', TODAY() ) != 1

AND

DATETRUNC('month', TODAY() ) = DATETRUNC( 'month', [Trip Date] )

)