I want to allow my users to only select either the 14th or the 28th of any month when they make a selection.
Is this possible?
Hmm, that's a good question. I had a look around for 10 mins and didn't find something that matches up with what you want, but I did find something you could try to tinker with.
@Mike Smith, a well known contributor to this site, has an interesting post on Date fields - see here.
If it were possible to combine both of the formulae Mike has posted below then it could work; instead of selecting the last day and first day of the month, you'd need to insert your own values of 14th & 28th.
Also on the same web page is this - you can specify two values using the OR function. This could be complicated though, I searched around for something that would be flexible enough to allow a 'floating day date'. Calculations can sometimes be done on Date fields by using DD / MM / YY calculations to 'split out' the dayday / monthmonth / yearyear values. In your case you'd need to split out where dayday = 14 or 28.
Hopefully someone can suggest something that works right away, but I thought I'd offer a couple of avenues for further exploration.
If it doesn't have to be a date field, you could create a choice column instead - simply define the column values available as 14th and 28th.
Then if you need a column to indicate the month, just create a new column called month and add in the values to pick from.