1793: 33 George 3 c.13: An Act to prevent acts of parliament from taking effect from a time prior to the passing thereof.
WHEREAS every act of parliament, in which the commencement thereof is not directed to be from a specifick time, doth commence from the first day of the session of parliament in which such act is passed: and whereas the same is liable to produce great and manifest injustice; for remedy whereof, be it enacted; and it is hereby enacted by the King’s most excellent majesty, by and with the advice and consent of the lords spiritual and temporal, and commons, in this present parliament assembled, and by the authority of the same,
[Clerk of the parliaments to endorse on every act the time it receives the royal assent which shall be its commencement, where no other is provided.]
That the clerk of the parliaments shall endorse (in English) on every act of parliament, which shall pass after the eighth day of April one thousand seven hundred and ninety-three, immediately after the title of such act, the day, month, and year, when the same shall have passed, and shall have received the royal assent; and such endorsement shall be taken to be a part of such act, and to be the date of its commencement where no other commencement shall be therein provided.
Parliamentary Archives: HL/PO/PU/1/1793/33G3n21
Further reading: Wikipedia.