Using a poshformat (which, if the use case was general enough, might be formalized into a microformat.) [0] Probably leveraging the HTML time element.

Of course, that lets you specify the execution time, but unless you have a system consuming the format that is aware of the additional semantics beyond the base HTML semantics, specifying that won't do anything.

[0] http://microformats.org/wiki/poshformats

Given the current specs, I am not aware of any way to do it, but in theory, you could do something like:

    <div visible="after:5pm">
    hello world
    </div>