Previous practice had been to date the acts (and, thus retrospectively, their application) as the first day of the session in which they were passed.

Before the passing of this act, most acts of Parliament were ex post facto laws, meaning that they were deemed to have come into force on the first day of the session in which they were passed, because of the legal fiction that a session lasted one day.

For this reason, and also because the House of Commons had a rule by which decisions of the whole house could not be reversed in the same session,[2] most acts of Parliament would included a clause providing that "This Act may be amended or repealed in the present session of Parliament".

[3] This was acknowledged in the preamble to the act to be a "great and manifest injustice", as individuals were bound by laws that did not exist when they took certain actions.

Similarly, because Scotland had changed its (also Julian) calendar in 1600 so that its year began in 1 January, the English 'Union with Scotland Act (1706)' was actually passed shortly after the Scottish 'Union with England Act (1707)' – in the same March of the same year in modern reckoning.