Large session timeout number expires session almost instantaneously

[octocat-mona]

Kibana version:
6.4.0
Elasticsearch version:
6.4.0
Server OS version:
CentOS (Docker image)
Browser version:
Chrome 68.0.3440.106
Browser OS version:
Windows 10 Version1709
Original install method (e.g. download page, yum, from source, etc.):
Docker
Describe the bug:
Setting a large number like a month (2592000000) for xpack.security.sessionTimeout actually gives an almost instant session timeout message in Kibana: You will soon be logged out due to inactivity. Click OK to resume.

There seems to be no maximum defined in the documentation:
https://www.elastic.co/guide/en/kibana/6.4/security-settings-kb.html#security-ui-settings

Steps to reproduce:

1. Set environment variable in Docker Compose file: XPACK_SECURITY_SESSIONTIMEOUT: 2592000000
2. Set Elasticsearch url, username and password variables
3. Run Kibana
4. Login to Kibana
5. Wait a few seconds

Expected behavior:
No session timout warning shown
Screenshots (if relevant):

Errors in browser console (if relevant):
N/A
Provide logs and/or server output (if relevant):
N/A
Any additional context:
N/A

Blocks: #68885

[octocat-mona]

This is still the case in 7.0.0, no planning for a fix yet?

[legrego]

Linking to @jportner 's analysis of the problem from a duplicated issue: #64264 (comment)