That depends on how you define the clock. Are you defining it in terms of its physical makeup or in terms of its purpose. It seems that you are defining in terms of its purpose and function therefore anti-matter clock is a poor fit. And while I can't think of an oppossite off the top of my head that does not mean there isn't one.
In fact all of us could fail to think of an oppossite to a clock and that would still not justify the statement that there is no oppossite to a clock.
In fact if a person wanted to they could go out and create the oppossite of a clock assuming one doesn't already exist.
Based on the function and usage of a clock principle one would first have to come up with a definition of a clocks function.
Here goes:
A clock is that which is used to keep and measure amounts of time that have passed, or a device to tell at what point in a day one is currently at.
Thus the oppossite of the clock would be:
That which is used to lose track of measured amounts of time that have passed, or a device that fails to tell at what point in a day you are currently at.
Thus I conclude that the oppossite of a clock is SE4.
Or you could go for the super impossible oppossite definition:
That which is not used to keep and unmeasure amounts of no time that have not passed, or a non-device to not tell at what point in a day one is currently not at.
Maybe I should throw in a few more nots and nons? A non-day?
What is my point? My point is that how you define the oppossite of a clock is based purely on how you define what is a clock.