Travelling from the present to the past met with limited success, so the scientists bent their energies toward bringing subjects from the past to the present (this was called "trawling"; trawling is a nautical term where a boat lays down a net and floats along, scooping up anything it finds). Even this method was very hit-and-miss; there were several failed attempts (like the famous Judge Crater) before the whitecoats found Doc Tanner. He was the only success of the original program, but others were later successfully trawled or travelled on their own: Michael Brother was a trawl subject (Fury's Pilgrims), and Silas Jamaisvous was sent back in time to 1896, then later returned to Deathlands (Dark Emblem).
Note: Originally, only certain gateways were used as part of Chronos; they had a separate set of computers that could be used to switch the gateway between matter transference and temporal travel (Pony Soldiers).
In Dark Emblem, Silas reprogrammed the computers in a redoubt in Puerto Rico to enable time travel, but it is unclear if this is possible for any gateway, or if (more likely) he created a workaround for that specific one. A list of gateways that were used as part of Chronos include: Mohawk redoubt (upper New York state); Chicago; Virginia (exact location unknown); Dulce, New Mexico.