The time stone was glowing when Dr Strange made it appear, to give it to Thanos. Infinity Stones only glow when they're being used. Which means Dr Strange was receiving the stone from either the future or the past. My guess is the future. With Tony and Nebula being the two left on Titan, it's likely that the Time Stone will appear to Tony right after he saw everyone fade away. Tony will then have to figure out what to do with it, and how to do it.