Wrong ClientModified Timestamp

Splinter Can you share the code you're using to reproduce this, and the output you're getting? I just tried to reproduce this and /download is returning the same datetime I'm supplying to /upload in a simple test:

curl -vX POST https://content.dropboxapi.com/2/files/upload \
  --header 'Authorization: Bearer <ACCESS_TOKEN>' \
  --header 'Content-Type: application/octet-stream' \
  --header 'Dropbox-API-Arg: {"path":"/test_281598","client_modified":"2018-6-22T13:5:28Z"}'

# > POST /2/files/upload HTTP/1.1
# > Host: content.dropboxapi.com
# > User-Agent: curl/7.58.0
# > Accept: */*
# > Authorization: Bearer <ACCESS_TOKEN>
# > Content-Type: application/octet-stream
# > Dropbox-API-Arg: {"path":"/test_281598","client_modified":"2018-6-22T13:5:28Z"}
# >
# < HTTP/1.1 200 OK
# < Server: nginx
# < Date: Fri, 22 Jun 2018 17:21:40 GMT
# < Content-Type: application/json
# < Transfer-Encoding: chunked
# < Connection: keep-alive
# < Vary: Accept-Encoding
# < pragma: no-cache
# < cache-control: no-cache
# < X-Server-Response-Time: 989
# < X-Dropbox-Request-Id: 7d064252bd53bd317bb02605d8ed4fd9
# < X-Robots-Tag: noindex, nofollow, noimageindex
# <
# * Connection #0 to host content.dropboxapi.com left intact
# {"name": "test_281598", "path_lower": "/test_281598", "path_display": "/test_281598", "id": "id:25N5ksooX-sAAAAAAAMpmw", "client_modified": "2018-06-22T13:05:28Z", "server_modified": "2018-06-22T17:21:40Z", "rev": "733c4021eccc7", "size": 0, "content_hash": "e3b0c44298fc1c149afbf4c8996fb92427ae41e4649b934ca495991b7852b855"}

curl -vX POST https://content.dropboxapi.com/2/files/download \
  --header 'Authorization: Bearer <ACCESS_TOKEN>' \
  --header 'Dropbox-API-Arg: {"path":"/test_281598"}' 

# > POST /2/files/download HTTP/1.1
# > Host: content.dropboxapi.com
# > User-Agent: curl/7.58.0
# > Accept: */*
# > Authorization: Bearer <ACCESS_TOKEN>
# > Dropbox-API-Arg: {"path":"/test_281598"}
# >
# < HTTP/1.1 200 OK
# < Server: nginx
# < Date: Fri, 22 Jun 2018 17:22:13 GMT
# < Content-Type: application/octet-stream
# < Content-Length: 0
# < Connection: keep-alive
# < accept-ranges: bytes
# < etag: W/"733c4021eccc7"
# < pragma: no-cache
# < cache-control: no-cache
# < original-content-length: 0
# < dropbox-api-result: {"name": "test_281598", "path_lower": "/test_281598", "path_display": "/test_281598", "id": "id:25N5ksooX-sAAAAAAAMpmw", "client_modified": "2018-06-22T13:05:28Z", "server_modified": "2018-06-22T17:21:40Z", "rev": "733c4021eccc7", "size": 0, "content_hash": "e3b0c44298fc1c149afbf4c8996fb92427ae41e4649b934ca495991b7852b855"}
# < X-Server-Response-Time: 1497
# < X-Dropbox-Request-Id: 0ff75dbf9fb1eab7c7ecefbf2c5ce379
# < X-Robots-Tag: noindex, nofollow, noimageindex
# <
# * Connection #0 to host content.dropboxapi.com left intact

So, it looks like this is working as expected at the API level, but perhaps you're using an SDK or library and there is a time zone issue at that level. If so, please make sure to include the SDK/library name and version too. Thanks!