To repro:

Create a document, then update it as follows:

POST my-index/my-type/1/_update
{
   "script" : {
       "inline": "ctx._source.timestamp_now = ctx._now; ctx._source.epochms = new Date().getTime();",
       "lang": "painless"
   },
   "upsert" : {
       "foo":"bar"
   }
}

In the results we're seeing, epochms is correct, but timestamp_now seems to be a random time. The ctx._now time is consistently increasing, but on various different nodes it has returned times ranging from 2003 to 2019.