No harm will be done using out of date toothpaste so it's safe to use out of date toothpaste even if it's way past the expiration date of 12 to 18 months.
But out of date toothpaste can lose it's fluoride stability and the ingredients in the out of date toothpaste my crystallize and separate out and the flavor may not be as good as it was before the toothpaste expired.