Changing Milestone Start Date changes the number of open issues on the chat

Summary

Changing the start date on a milestone to a more recent date seems to change the number of open issues.

Steps to reproduce

1. Go to a Milestone on the milestone page and note the number of open issues on the chat.
2. Change the Start Date to a more recent date.
3. Check the number of open issues again on the Graph.

Example Project

https://gitlab.com/mflabs/Cavalry/milestones/3

What is the current bug behavior?

Number of open issues on a particular day increases

What is the expected correct behavior?

Should remail the same

Relevant logs and/or screenshots

Output of checks

This bug happens on GitLab.com

Results of GitLab environment info

Expand for output related to GitLab environment info

(For installations with omnibus-gitlab package run and paste the output of:
sudo gitlab-rake gitlab:env:info)
(For installations from source run and paste the output of:
sudo -u git -H bundle exec rake gitlab:env:info RAILS_ENV=production)

Results of GitLab application Check

Expand for output related to the GitLab application check

(For installations with omnibus-gitlab package run and paste the output of:
sudo gitlab-rake gitlab:check SANITIZE=true)
(For installations from source run and paste the output of:
sudo -u git -H bundle exec rake gitlab:check RAILS_ENV=production SANITIZE=true)
(we will only investigate if the tests are passing)

Possible fixes

(If you can, link to the line of code that might be responsible for the problem)