Hi Sarah,
Thanks for posting in the Community!
Thank you for reporting the unexpected behavior you've encountered when recording GST payments. There is a reported Product Investigation under INV-109291 for Unable to record GST payment with the correct date.
I recommend contacting our Customer Care Team. That way, they can add your company file to the list and you can get an updated notification once a fix has been deployed.
Post again in the Community if you have other questions.