All versions of mongoimport were able to parse and import following simple document containing timestamp:

{ "ts" : { "$date" : "2019-07-25T20:25:15.202+0800" } }

Now if I try to import this document using mongoimport version 4.2.0-rc4 it will print this error:

2019-07-26T18:30:06.927+0800 connected to: mongodb://localhost:27017/
2019-07-26T18:30:06.928+0800 Failed: invalid $date value string: 
2019-07-26T18:30:06.928+0800 0 document(s) imported successfully. 0 document(s) failed to import.

The problem is that new mongoimport only accepts date strings with colon in the timezone offset part. So following document will be imported successfully:

{ "ts" : { "$date" : "2019-07-25T20:25:15.202+08:00" } }

I believe this is a regression in mongoimport functionality comparing to previous versions.