Countdown

The cookie is signed using flask. The page says 'Time is key'.

Using the flask-unsign utility, we can recover the secret key, 'Time'. We then resign data to move the 'end' date forwards.

flask-unsign --sign --cookie "{'end': '2020-06-13 16:59:59+0000'}" --secret 'Time'

Sending the resulting cookie gives us the flag.

rgbCTF{t1m3_1s_k3y_g00d_j0k3_r1ght}