Welcome to the forums, by the way.
There has been speculation about the date on the headstone. Some theories are that since the parents know what happened, they also know WHEN she died, and it could have been around midnight, still the 25th. This is in keeping with the estimated time of death of midnight - 1 AM, which is based on the arrival home around 9-10 PM, the location of the pineapple in her digestive tract (food empties from the stomach into the small intestine, where the pineapple was found, in about 2 hours. It is also based on the stage of rigor mortis she was found in- FULL rigor, which takes about 12 hours in indoor temperatures. So counting back from the time she was "found", 1 PM on the 26th, that also puts the time of death in that same midnight - 1 AM window.
The coroner never performed the tests for Algor Mortis, but both rigor and algor mortis proceed in scientifically predictable fashion. Like the digestion of the pineapple, science doesn't vary according to whether you believe the parents are innocent or not or believe rubbish about the pineapple having been eaten earlier in the day.
There are some conditions that affect how the body loses heat after death (rigorous exercise, VERY high fever, and pontine hemorrhage (which affects the area of the brain that controls body temperature regulation) but JB did not fall into any of these categories. The basement would still be considered "room" or indoor temperature. And actually, JR had made comments that their basement was actually warmer than average because of the furnace, and stated it as a reason why they never rushed to repair the broken basement window that he admitted breaking himself. If the room was warmer than average, and clothed and wrapped in a blanket, that could actually lessen the time needed for full rigor to form, making her time of death even closer than the 1 AM estimate. Then- the events that happened occurred right after returning home- the pineapple snack included, and there can be no doubt that the the parents were awake and up and about when she was allegedly "kidnapped"- something that simply could not happen. This was no "first-floor open window child snatching". These "intruders" had to have all but met the parents in the house. THAT is why the parents HAD to lie about the pineapple. If they admitted feeding it to her (someone did, because her prints are not on that bowl- Patsy's ARE) then that means they lied about her being awake when they got home. And it also puts the death occurring at a time when they HAD to have been awake and could certainly have been before midnight, when it was still the 25th.
Some people feel they wanted that date because it was more "special" than the 26th. Some feel it was because that was the last time they claimed to have seen her alive.