The good/bad old days
Definition 1 of 2
Pro Tip 1/3Nostalgia Trigger
The phrase often expresses a yearning for the past when things seemed simpler or better. 
Grandma loves recounting stories from the good old days of her youth.
Pro Tip 2/3Positive Emotion
It conveys warmth and fondness for the past, even if it wasn't perfect. 
Those were the good old days, we didn't have much but we were happy.
Pro Tip 3/3Cultural Reflection
May refer to times reflecting cultural or social values of a past era. 
The good old days of rock 'n' roll often come up in music conversations.