Diesel Tuning Fundementals - Injectio timing. Why 120?

"A really handy calculation to always keep in the back of your mind is that we can quickly calculate the cycle time of the engine or in other words how long the engine takes to complete one full cycle by dividing 120 by the current RPM."

why 120? i did the math on the next section with the injection timing and it made me realize it is all top dead center which makes no sense at all. do the 120 for RPM 1k-4k and it will always lead back to TDC which made the toyotas timing confusing as all hell. to make it clear what im talking about since i ramble a lot; with 120 all RPM values math back out to TDC when converted to injection timing degrees