The digital clocks that did not feature a delay or increment function did exist as early as the 1970’s.
I came into chess in 1988. At the time the technical “best” was the Master Quartz which was an analog clock with Quartz precision. It was an electronic clock versus a mechanical one. I did see a Kaisha clock at a tournament, but it was more of a novelty than the standard.
It was in 1996 when the first delay clock came on the market here in the United States. This was the Excalibur GameTime. I know this because I was one of the very first to buy one. I remember calling the USCF number and trying to order one, just before they were available. Also, it was identified as the USCF GameTime clock and the Excalibur name was on the box but certainly not prominent in the advertising or marketing.
I recall the DGT clock coming available next and then the Saitek, but I am aware of the Chronos most likely also being available in the last half of that decade also. Oh yes, I did own each of these clocks as well.
I did own one of the Saitek clocks that had the Bronstein feature. It added the delay time to the clock after you would hit your button rather than a true delaying of your clock starting, which is now known as the USCF Delay style.
I believe it was in about 1999 or so that I bought my first Chronos. I sold my GameTime to buy it.
Today, I own 3 Chronos clocks, 2 Excalibur GameTime II clocks and one DGT XL Clock that goes along with my DGT board.
In my opinion, the Chronos is the standard of the chess clock of today with its durability and quality display. The Excalibur is the best buy for the money. It functions very well and its display is great. The only things I prefer about the Chronos over the Excalibur is the body of the Chronos is aluminum versus plastic and the Chronos has an option to see the minutes and seconds when the time is above 20 minutes, where the GameTime II only shows this under 20 minutes. But seeing that you could by 2 or 3 Excalibur clocks for every one Chronos, the Excalibur is the best for the money spent.
I also like the Saitek Competition Pro III. While it doesn’t have the time set options of the Excalibur, the green led lights in the buttons are a very nice touch. I owned one of these and sold it with some other chess equipment. I then bought the Excalibur clocks about 8 months later because they were a lot less expensive ($25 for the Excalibur versus $39 for the Saitek).
I have seen the new DGT clock that is portable and find the display to be a bit small and there is no way to see which side is to move by the buttons, led lights or anything except to look closely at the display to see the numbers changing.
So, in the market and history of Digital Clocks, I feel the delay clocks to be the most prominent. It started with the Excalibur/USCF clock in this country. 13 years later, we see a vast number of companies and different clocks available and at very reasonable prices as well.