A Time-Clone is a duplicate of an individual created by warping the fabric of space and time. The process is only known to be effective in creating duplicates of robots, most notably those from the reality of Earth 20XX.
Using the reality-bending properties of the Skull Egg Zone and the Wily Egg, Dr. Eggman and Dr. Wily managed to create Time-Clones of Wily's Robot Masters, most of whom became the Robot Master Army. The Copy Robot and Genesis Unit were also Time-Cloned and sent to attack the party of Mega Man, Sonic the Hedgehog, Proto Man, Tails, and Rush after they entered the Skull Egg Zone. Proto Man and Mega Man were the first to recognize that an unusual process was being employed, as robots from their world had only previously returned from complete destruction due to the power of Ra Moon. The Genesis Unit later returned as part of the army, who engaged the heroes and their expanded force of allies outside the Wily Egg. While the heroes managed to hold their own, it quickly became evident that the doctors could continue to create more Time-Clones to dispatch against them. As such, Mega Man and Sonic led a small party to invade the Wily Egg, where they were forced to engage Time-Clones of the Mega Man Killers. (SU: #52; StH: #250; MM: #27)
Sigma-1 later employed his own Time-Cloning process in his base on the Lost Hex, by combining the Genesis Portals with the Unity and Master Engine energy to create duplicates of fallen and future Mechaniloids and Mavericks. In the case of the latter, Sigma discovered that the process was not a perfect duplication, as the personality of the Mavericks didn't remain intact. However, when it became clear that they retained their loyalty to him, Sigma proceeded to clone an entire army of Mavericks. The Mechaniloid Time-Clones faced the heroes and were defeated, but after a brief scuffle caused by the Deadly Six the heroes were forced to face the Maverick Army. While Axl and his comrades recognized various old foes and friends amongst the enemy, Zero noted that there were absences, such as Colonel and Iris. (SU: #77; SB: #9; StH: #274; MM: #51)