It looks like both years have validity. From the Scottish Tartans Authority:

"the Act of Proscription 1747 which was actually introduced in 1746 but gave the authorities a year to prise all arms from the Highland miscreants. August 1st 1747 was the 'crunch' day for the Highlanders "

I don't remember which site I got the '46 date from, but I guess they used the introduction date. Either way it's August 1.