Extract Sub string and reverse

I have date & time filed in the log as below, which I have parsed to a filed in the logstash.
11-12-2015 10:00:00,301

However I want the date only in 2015-12-11 (YYYY-MM-DD) format into a variable, say event date in logstash.

How Can I extract the date and convert into YYYY-MM-DD (or reverse the string) and store into a variable.

If you want to parse it into a complete timestamp use the date filter, otherwise use the grok filter to extract each component of the date into temporary fields and a mutate filter to put them back together in the desired order. Something like this:

grok {
  match => ["name-of-field", "^%{MONTHNUM:month}-%{MONTHDAY:day}-%{YEAR:year}"]
mutate {
  add_field => {
    "event_time" => "%{year}-%{month}-%{day}"
  remove_field => ["year", "month", "day"]