Please add your vote and opinion to this topic in the official Adobe feedback forum: Lightroom: A better search/find in the library module (Library Filter). Meanwhile, as a workaround, you could use the Any Filter plugin, which provides that capability.
I concur with John - AnyFilter is a great way to find photos based on any criteria existing in Lightroom .
That said, another option is to use the Metadata Extensions plugin (it's free, and I wrote it), which includes 'Hour of Day' item for library filters or smart collections.
Which should you prefer?
* AnyFilter is faster for one-time/on-demand queries, and more powerful in general, but relies on a proprietary plugin dialog box for it's human interface.
* Metadata Extensions takes longer for a one-time initialization of the metadata in Lightroom, but then allows you to use "standard" (built-in) Lr search & filter mechanisms afterward.
So, if you frequently want to filter or smart-collect based on time (hour) of day, and/or you don't use AnyFilter often, then you might find Metadata Extension is most convenient.
If however, you use AnyFilter frequently enough that you've become accustomed to it's UI, and/or you don't filter/smart-collect on time-of-day that often anyway, AnyFilter may be preferable.
Which do I use?
* Both, depending..
PS - another option is Jeffrey Friedl's DataExplorer plugin. I don't have enough experience with it to comment, but it does support hour-of-day.