Time Warp signs are slightly smaller versions of signposts. They are also colored yellow, grey and black. Their sign reads either "Past" or "Future" with a yellow font. The symbol of signpost can be seen at the left hand corner next to Lives hud after moving past through a Time Warp sign.
Sonic the Hedgehog CD
In Sonic the Hedgehog CD, running past the Time Warp sign in gameplay makes the signpost flip once while a voice states the particular signpost's name at the same time. Afterwards, the player must build enough momentum to break the sound barrier while the signpost's HUD symbol flashes. When the player gains enough speed, the screen flashes and transports the player to another time. If the player fails to build enough speed, the symbol disappears and the player must try again at another Time Warp sign. It will also disappear if the player runs past the Goal Signpost before the Bonus Plate. When the player is transported to the past or future, the player's life icon is replaced with a P or F icon, representing Past and Future, respectively.
Time Warp signs appear in the second Zones of each stage. In present time, both past and future signs exist and are available to use. However, if the player is in the past or future, only one type of sign will exist. The player can either travel to the bad future (if the player has not destroyed the all the robot transporters in the past or has not collected all the Time Stones), or the good future, if the player has collected all Time Stones from the Special Stages or destroyed all the robot transporters in the past. However, if the player has destroyed all the robot transporters, the player will automatically transport to the third Zone's stage in that level with the good future (or the bad future if the player has not destroyed the robot generator from either of the first two stages) where Time Warp signs are not featured.