At the simplest level a time hit is nothing more than a stop hit with opposition. But like most simple things, it is not really simple. Stop thrusts and stop cuts rely on timing for success and profit from being direct, fast actions that avoid the opponent's blade. The problem is that you must rely on one of a number of things happening that make your stroke successful:
(1) you successfully steal the time so that the right of way is yours - foil and sabre
(2) the opponent physically stops his or her attack on the impact - foil, sabre, or epee (this does happen if you have exactly the right psychological moment, but rarely with experienced opponents)
(3) the opponent missing - foil, sabre, or epee
(4) landing early enough that you lock out the opponent's action - sabre and epee
The correctly executed time hit solves all of the right of way and timing problems because you control the opponent's blade so that there is only one light. One light solves all sorts of issues of fact (if you know the difference between matters of fact and those of the rules you understand what I am suggesting).
So if this technique is the holy grail of not being hit, why don't more people do time hits? The simple answer is that they are difficult to do correctly. To even a greater extent than a stop hit a time hit is truly an attack on the opponent's attack. You have two objectives which must happen at the same time - hitting the opponent and intercepting and controlling the blade. As the opponent starts his or her attack you must discern the final line of that attack and simultaneously attack into the line. As you do you must not just close the line, but close the line capturing the opponent's foible with your blade and bell, displace it from the line, and maintain that control through the action. Only closing the line does not provide assurance that the opponent will continue the attack into the closed line.
How you close the line to capture the blade is critical. If you start by closing the line and then move forward with your blade the opponent may well see what you are doing, and, if they are fencing eyes open, deceive your attempt to take the blade. Your goal is to intercept their blade and shove it off line relatively later in the action so that you apply leverage as you accelerate for your hit.
Tactical application is difficult. The Masters of the Classical Period taught time hits against compound attacks - that requires a high level of technique, perfect timing, and a perfect understanding of the opponent's attacking technique. I am happy if I can do a time hit against a simple action. In foil and epee - 8th works well (especially as an intercepting action against a slow disengage), 6th and 4th are possible. In sabre, time cuts in 3rd work, 4th is more difficult, but 2nd is possible.
Incidentally, if you read fencing manuals from before World War II, stop actions may be called time actions and time actions may be called stop actions. The late 1800s and early 1900s have a certain amount of turbulence in terminology, and reading how the technique is described may be necessary to understanding the author in modern terms.