The date fields are used in searches to let the user specify a date when an event took place and exclude churches that didn't exist then. We don't always know the exact years when the churches were founded or closed so we try and specify a reasonable inclusive range, and modify things as were learn more about them.
It isn't easy to get dates in many cases but there are a number of techniques we can use to help get a range of years. The best starting point is record office listings of deposited registers. The founding date must be before the date of the earliest register, so supply as the founding date till we get something better. The text that appears as a result of searches is then
before nnnn so the user is not mislead into thinking this is the actual founding date. Note that we should be able to list churches where events may have taken place, not just those where the records exist. One thing to watch out for is if a church has moved from one location to another. The records will cover both locations and so that has to be taken into consideration
Another good place to find the founding date is in old directories especially for the very oldest as the directory will typically give some of the history and the accepted date of founding. If a church is listed in a directory, then the founding date must be before the date of publication when no founding date is given in the directory entry. There is history available for most churches - e.g. in Pevsner's Buildings of England. Finding the appropriate publication may limit this approach.
Another source is maps, if you find the church on map then it must have been founded before the date of the map. Be careful to ensure that a full description of the church appears on the map. There is normally enough space for this in rural areas, but not in towns and cities. And don't make assumptions, e.g if it just says 'Methodist chapel' that's rather too vague. And remember that changes may have taken place after the survey. But if you have corroborating evidence such as another source that the particular church was there later, then that is good evidence to make the founding date earlier.
Those are some good techniques for dealing with founding dates but what about finding out if a church is still in use? Nowadays there is usually a diocesan website which hopefully lists all the churches in the diocese. If I find them on the website I assume they are still open . For those churches that are mentioned you have the bonus of another piece of information, a URL which we store and use to provide a link. There is one thing to watch out for though is how they deal with closed churches, especially in the cities. It does seem to be a fashion to add the dedication of a closed church to the one the remaining congregation have combined with. But don't assume that it is the last name that has closed, they sometimes have the closed church as the first saint's name.