How to actually get the date change to work:
Pandaman822: "March 2019 is the last month where you only need to do ~9-10 challenges to get gold. April onwards is when it takes significant more challenges to get badges."
Set your phone date to the last day of the month. Then, complete all of the challenges to unlock:
Then, set your date to the last day of the previous month. Complete challenges until you unlock Gold.
Repeat for each previous month until you unlock:
Then, get up to Silver for the next few months until you unlock:
Then, just reach Bronze for the last 2 months which will unlock: