creme fraiche 1 month out of date!
Crème fraiche is a cultured product, meaning it has lots of helpful bacteria that cause the initial sourness.

These products will tend to become progressively more sour over time - in the case of crème fraiche, more like sour cream - but the pre-existing bacteria tend to compete with (and win against) other parasites, such that the food will start to smell "off" long before it is unsafe, and may actually never spoil as far as bacteria are concerned.  It may, however, develop mold.

If it's not moldy, and it smells fine to you, then feel free to eat it and don't fret about it.  Sell-by dates are indicators of quality only, not food safety.