Hi James,
I know it's an old thread but I was looking for the same info and managed to understand how it works from the source code in tvheadend.js (function tvheadend.toCustomDate), so the available options are :
M = month. repeat the letter for different variants : %MM (2 digits) , %MMM (month name short), %MMMM (month name long)
m = minutes. use %mm for 2 digits
s = seconds. use %ss for 2 digits
S = millisecond
y or Y = year
d = day. repeat the letter for different variants : %dd (2 digits) , %ddd (weekday short), %dddd (weekday long)
h = hours. use %hh for 2 digits
q = quarter
Cheers