Marking 'before' the baptism date is not necessarily correct if one were to be technical rather one can infer that it was 'on or before.' Unfortunately, that is not an option so one would be technically correct only by filling in the next day's date and selecting 'before.' For the record, I DO NOT do that. I do exactly as others suggest by using the baptismal date and marking it 'before'.
I can also offer these thoughts:
1) The proximity of the birth to the baptism has always been a cultural thing with the geography and the era being factors. Mid-19th century baptisms were mostly within a day or two of birth and I have seen so many that were the same day that one would would not likely conclude these were 'emergency baptism.'
2) On the other extreme, I have seen octogenarians being baptized.or entire families being baptized the same day. These were usually related to conversion from one faith to another.
3) Sometimes, the 'official' civil date of birth is AFTER the baptism date. In these cases, the date of birth was almost invariable misrepresented by the parents in order to avoid a fine for not reporting the birth within the period proscribed by law.
4) A similar problem exists for burial dates. One would certainly hope that the burial was after death (though not always the case, which is why we have the term 'dead ringer'). It is typically safe to assume that the burial was within a few days of death although there are certainly cases where a body had been lost for a long time and then buried after being discovered.