Units are created by particular unit-producing buildings in Rise of Nations. This page describes how long it takes for units to build. The basic unit of time in Rise of Nations is a frame, typically 1/15 of a second at normal game speed. In unitrules.xml, each unit has a JOB_TIME value. This value, multiplied by 1.2, is that unit's base build time in frames. For every one of that particular unit already created, the base build time is increased by 7.5 frames. On every frame, the current progress increases by 1. If the current progress, subject to any potential multipliers, equals or exceeds the total number of frames needed to build the unit, then on the next frame the unit will be created (assuming it will not exceed the population cap).