One other thing with my example above, having entered 28 Jan 1587/88 and FTB converting it to 28 Jan 1585/86, for some unknown reason, then when you export the GEDCOM file it comes out as 28 Jan 1586/87.
The date I entered is a perfectly valid date according to the GEDCOM 5.5 standard and should be accepted as-is.
Is FTB attempting to convert the date to New Style assuming that I'd entered an Old Style or vice-versa?
With regard to your work around, when entering a free text date, a date phrase in GEDCOM terms, the GEDCOM file export is not enclosing the date in the parenthesis required by the standard.
I've spotted another work around using the free text date option. I can enter it as an 'interpreted' date as INT 28 Jan 1587/88 without the parenthesis date phrase following it. This exports in GEDCOM correctly although it does appear that a date phrase is required to follow an interpreted date by the GEDCOM standard.