How can I make sure the timestamp is properly parsed?

Well, compare the logs!

If we can parse your timestamp, you will notice that the timestamp on your log is the exact timestamp on kibana.

For example: 

(please note the +0000 section that indicates UTC, and my kibana is at GMT+2, hence the two hours time difference)

And if we can't extract timestamp from the log, we will add a timestamp to it as soon as we received it, to be as accurate as possible, but it may be a while depends on your shipping method, bulk size etc..

It will look like that:

Timestamp extractions depends on the log type, so make sure you shipping one of our out of the box supported formats, or that you have contacted us to get a custom parsing.

 If you are sending your logs as JSON's, just fill your timestamp in the @timestamp field, with that format:

yyyy-MM-dd'T'HH:mm:ss.SSSZ

e.g.
2016-01-06T12:14:00.000Z