The shelf life of marshmallow cream is around 1 year past the use by date.
As long as the marshmallow cream remains sealed and stored in a cool dry place such as in the pantry the marshmallow cream should last for at least a year past the use by date.
The use by date doesn't mean it's bad but after awhile the marshmallow cream loses it's flavor and is not as good tasting but it's okay to eat it.
As long as the marshmallow cream has no bad or sour smell to it and no mold growth in it then the marshmallow cream is okay and safe to eat.
If the marshmallow cream smells bad or tastes bad then throw it out.
Once opened you can freeze marshmallow cream to extend the life of the opened marshmallow cream.
Marshmallow cream will freeze just fine and when in the freezer the marshmallow cream will last for 3 months before it starts to lose it's quality.
So eat the frozen marshmallow cream within 3 months of freezing it for best results and taste.
Also be sure to try and get as much air out of the marshmallow cream container before freezing it and use the marshmallow cream up before 3 months.
Because the longer the marshmallow cream is frozen the less tasty it gets because it loses it's flavor overtime.