This looks like epoch time to me. If it is, then Splunk will figure it out properly. You could also put this in props.conf
[yoursourcetypehere]
TIME_FORMAT=%s%3N
If you want to give Splunk a little more help, you might add a TIME_PREFIX
setting and MAX_TIMESTAMP_LOOKAHEAD = 20
Refer to Configure timestamp recognition in the manuals for more info.