Butter that has gone bad will have a rancid smell or sour smell to it and the butter that has gone bad may also have mold on it and the butter may get hard and crumbly as well.
Expired butter is safe to eat unless the expired butter has actually gone bad.
The expiration date on butter does not mean it's actually bad or unsafe to eat and as long as the butter smells good, tastes good and has no mold, maggots etc in the butter then it's safe to eat.
Most butter lasts up to 6 to 9 months in the fridge before it goes bad.
You can freeze the butter to make it last longer and when frozen the butter will last for up to a year.
However when frozen the butter loses it's texture and the taste can change as well.
Some signs that butter has gone bad is a rancid smell or sour smell to the butter and the butter may also become hard and have mold growth on it as well.
If the butter smells rancid, sour or has mold on the butter then you should throw it out as it's no longer safe to eat.