[SOLVED and THX]"ISO8601" in date filter

I am working with a log that have two individual fields for time and timezone offset (tab separated) like the following:

2014-12-14 23:59:40.227 -8

I was able to use mutate and add_field to make them into one new field(timestamp) looking like this:

2014-12-14 23:59:40.227-0800 or this: 2014-12-14 23:59:40.227-08:00 or this: 2014-12-14 23:59:40.227-08,

and none of them was recognized by ISO8601 pattern in date filter, the assigned timestamp displayed eventually is

2014-12-15T07:59:40.227Z which is off by 8 hours, so my timezone offset was somehow never accepted, but the

time was. I've also tried to write my own Joda pattern: YYYY-MM-dd HH:mm:ssZZ which did not work either.

Anyone could tell me how I can adjust my timestamp field or Joda pattern to make date filter recognize it? Thank

you for the help!