Millisecond with decimal time format

My nginx log uses timestamp $msec, with resolution up to 0.001 second. What ES 2 time format shall I use to load it? I am trying not to multiple it by 1000 and then load to ES.

It seems others use the decimal seconds too. eg: when I send collectd metrics json to elasticsearch, I see


It would be great if ES can support this Unix Epoch time down to 0.001 seconds.