I'll put this here incase someone finds it useful.
The problem is that some time formats are not supported.
Taking a look at the sutime/english.sutime.txt file, you'll see a line like those seen below. The TODO there shows other formats can still be added. I added 2 others to mine as seen below:
# TODO: Support other timezone formats
{ ruleType: "time", pattern: /yyyy-?MM-?dd-?'T'HH(:?mm(:?ss([.,]S{1,3})?)?)?(Z)?/ }
{ ruleType: "time", pattern: /yyyy-MM-dd/ }
{ ruleType: "time", pattern: /'T'HH(:?mm(:?ss([.,](S{1,3}))?)?)?(Z)?/ }
#The entries below are newly added to support other time formats.
{ ruleType: "time", pattern: /dd\/MM\/yyyy/ }
{ ruleType: "time", pattern: /dd-MM-yyyy/ }
The newly added entries enable SUTime to correctly identify time formats of the form:
20-12-2014 or 28/12/2014
which is identical to the OPs required form.