RANKX is not sequential

Here's a measure that does the ranking on the fly:

[Index] =
if( HASONEVALUE( Transmission[Tx_Date] ),
	RANKX(
		ALLSELECTED( Transmission ),
		Transmission[Tx_Date],
		SELECTEDVALUE( Transmission[Tx_Date] ),
		ASC,
		Dense
	)
)

The above is relative to the selected dates. If you want to have an index (measure) that's absolute against the whole Transmission table, then you should use this:

[Index] =
if( HASONEVALUE( Transmission[Tx_Date] ),
	RANKX(
		ALL( Transmission ),
		Transmission[Tx_Date],
		SELECTEDVALUE( Transmission[Tx_Date] ),
		ASC,
		Dense
	)
)

If you want a calculated column, then... your formula works correctly, which I've checked. If you have a problem, then it most likely means your data type is not correct. Make sure you're using THE CORRECT DATA TYPES.

Best

D