You can cheat a bit to get what you want ;-)
You just have to convert frames into seconds : time = frame/FPS
However, the frame duration are not fixed by default. You have to set the "Process Mode" of the animation to "Fixed" (from the inspector or via set_animation_process_mode(ANIMATION_PROCESS_FIXED)
)
It means that the animation will play at the same FPS than the Physics of the game. You can find this value in the project settings/Physics/fixed_fps or Globals.get("physics/fixed_fps")