Yes and No.
The registry is not the only place where a program can and will peg the time of install: It can be placed in a irrelevant looking file somewhere in windows system folders, written to an existing file, written to a specific sector in the boot drive, and often in more than one place, for tamper detection. Also, programs may mark not only time of install but also time of first run as the beginning of the trial.
So simply monitoring the registry will not work but in the simplest protections.
there are 2 other avenues to explore: Install monitors: will give you a catalog of all the reg-keys, files and modifications to existing files that a installation did, and even some allow you to "wipe" or Total Un-install" your app, by erasing all traces left behind by the installation. works in some but not all the cases.
Simpler and lazier, if you don't want to unpack, trace and reverse secured applications, is the use of virtual machines, available from M$ and VM ware:
Install an OS in a virtual machine, tweak it to your taste, save a copy as an archive. Now clone it, and install your time limited application in the clone.
Every time it expires, dump the clone, make a fresh one from the archived OS master, reinstall the time trial, go for another 30 days (The closest a computer will get to a box of Kleenex

).
The simplest and laziest is to buy the damn application.