A Long Day with Time Floodgate is the 1st chapter of Doraemon Volume 31.
Nobita has to finish his homework first before Tamako allows him to visit Shizuka, so Doraemon aids him with one of his gadget. However, troubles quickly arise when it is misused.
Tamako refuses to allow Nobita to leave the house unless he finished his homework, despite the fact that he has to go and play with Shizuka at 4pm. As Nobita begins to worry that Shizuka will get angry and never marry him, Doraemon decides to help out with "Time Floodgate", a gadget capable of slowing down the passage of time. With the help of the gadget, Nobita successfully finishes his homework and still has time to have fun with Shizuka, much to his pleasure.
Intrigued with the gadget, Nobita decides to borrow it despite Doraemon's plea, as the gadget's power can cause big trouble for everyone. Before he can stop Nobita, Doraemon is dragged by Tamako to the garden as it today is his duty to weed the lawn. Since there are so many weeds, Tamako suggests Doraemon to weed only for 2 hours until 6pm.
Meanwhile, Nobita begins to use Time Floodgate to extend the 1 hour that he gets to play which Shizuka, who is having a piano lesson at 5pm, causing trouble for Doraemon who is forced to keep weeding the lawn due to the extension of time. After returning home, Nobita uses the gadget once again to extend the "Space Tarzan" program so he could watch it a bit longer but eventually falls asleep, causing even more trouble for the actor(who has to keep fighting the monster), Doraemon, Shizuka (who is having a continuous piano lesson), and Nobisuke (whose work hours never end).
After Doraemon manages to weed the entire garden, he locates Nobita and tells him to disable the Time Gate, allowing 6pm to finally arrive and free everyone from their troubles. The chapter ends with Doraemon contemplating on whether or not to tell everyone of Nobita's misdeed (and let him be punished).