Such relation back or forward contravenes no principle of law and is determined by the intent of the parties as deduced from the instrument itself.” As a practical matter, the proper date to put on an agreement is something that corporate counsel is likely to have to make a judgment call on quite often.
This is because documents take time to draft, negotiate and execute.
For those with an hour to kill thinking about the issues, Jeffrey Kwall and Stuart Duhl wrote an excellent article on backdating that was published in Business Lawyer in 2008.
The Court considered the construction of the whole policy and held that the exclusion clause took effect on the backdated date chosen by the parties.
In the US, however, there seems to be have been much more consideration of the issue (at least according to my Google search results).