We were having so much fun laughing about the good old days that we lost track of time.
We were talking and laughing about days gone by and before we knew it we forgot all about the time.
どちらも、「昔を懐かしんで、時間を忘れて話に花を咲かせた。」というニュアンスです。
have so much fun 「大いに楽しむ」
good old days で「古き良き時代」
lose track of time 「時が経つのを忘れる」
days gone by 「過ぎ去った日々」
ご参考になれば幸いです！
日本 
