When I create a Sales Invoice with a Due Date of 4 October 2015 it appears on the Invoice as 3 October 2015. All other dates have worked correctly. Is it me or this a bug?
I don’t know why this happens to you. I just tried it and 4 October 2015 (a date in the past) works fine.
Thanks Tut.
I don’t know why either.
I have changed it from “by” to “net” & calculated the right number of days.
This then displays 4 October 2015 as required.
This is something for @lubos to look into. If it can happen to one user on one date, it could happen to anyone anytime. I suspect the reason is something obvious that will make both of us feel sheepish. 
I’m able to reproduce this issue but the bug appears to be in the 3rd-party library.
It seems like if the date format is dd/mm/yyyy and trying to select 4th October in a year where this date falls on Sunday, the date will be parsed as 3rd October, Saturday.
The next time this problem should happen again will be in 2020 so I’m pretty sure by then the problem will be fixed.
The workaround for now is to switch date format to mm/dd/yyyy under Preferences, then you will be able to enter 4th October 2015 fine. Then you can switch back your date format to original settings.