I know I can hardcode or create my own constant to represent either 20tick/sec or 50millis/tick, however are either of these constants already public defined somewhere either in the net.minecraft or net.minecraftforge code?  I would rather depend on a value provided from a greater authority, rather than hard code my own constant based on *public* common knowledge.  You know, just in case a future minecraft/server alters the TPS value...

I've never seen one. If you truly cared about real-time seconds you could use the system timer functions in Java, but most people believe that any time-based activities in Minecraft should lag together with everything else (which I agree with) so yeah just divide by 20.