It seems MongoHub does not take milliseconds of a date into account, hence, opening and saving a document, without modifying it, will clear out the millis of a date.

Steps to reproduce:
Save a document with some $date field
"someDate": { "$date": 1384297199999.00000 }

Read it back out, it shows as
"someDate": { "$date": 1384297199000.00000 }

Save it, and milliseconds are lost. The date is now in fact 1384297199000.00000 i mongodb.