Kronos is an operating system with time-sharing capabilities, written by Control Data Corporation in 1971. Kronos ran on the 60-bit CDC 6000 series mainframe computers and their successors. CDC replaced Kronos with the NOS operating system in the late 1970s, which were succeeded by the NOS/VE operating system in the mid-1980s. The MACE operating system and APEX were forerunners to KRONOS. It was written by Control Data systems programmer Greg Mansfield, Dave Cahlander, Bob Tate and three others.