Milestone duration: error message is not accurate enough

Summary

(Summarize the bug encountered concisely)

Steps to reproduce

  • Go to the milestone creation screen (Issues/Milestones/New)
  • Fill fields
  • Chose a Due date that is the same of the Start date
  • Hit Create milestone

What is the current bug behavior?

The error message displayed is:

Start date Can't be greater than due date

Capture_d_écran_2017-09-26_à_12_h_07_min_16

What is the expected correct behavior?

By entering two identical start and due date, the start date isn’t greater than the due date.

Depending on how it should behave (can the two dates be the same or not?), the message should be something more like:

Due date must be greater than start date.