I see so many comments where the date isn't included, doesn't include a year, is missing a digit, etc, so I thought I'd post to reassure you that you're generally on the right track.
But firstly, thank you everyone for letting us know, and I hope that those missing years or digits don't make the date task too frustrating!
A few people have commented on playbills with repeat dates (e.g. this one from Nottingham - Oswestry 1755-1848, http://access.bl.uk/item/viewer/ark:/81055/vdc_100022589132.0x000002#?cv=247&c=0&m=0&s=0&xywh=-103%2C292%2C3174%2C2655). We might find a way to flag those exceptions, but in the meantime, it's probably best to type in the first of the repeat dates.
This matches our request to only mark out titles for performances on the next date, not any future performances listed.
Please let us know if you have any questions or further comments!