It’s fine if the date is a hazily-recalled estimate. Usually the only way to find a more precise date is if a player happened to journal one of their early milkings, but not everyone does that. We care more that you have a hellworm now, because the present where you have Squishmallow is more important than the past where you didn’t!
Also I have some squishmallows on the surface and now I’m going to imagine that’s what all Hellworms feel like when you pet them.