You can display custom messages on the login screen and inside the desk on the dashboard visible to all users. The related permission within Role definition should be enabled to be visible on the dashboard.

Announcements can be added, edited or deleted by opening the announcements settings. This is located under Other Settings:

Clicking on the link opens a box where you can make your modifications.

An example entry is shown below:

1[showonlogin] = '1'
1[type] = 'warning'
1[heading] = 'Sample message Emails are Not working'
1[dateTime] = '18.Feb.2014 16:05'
1[text] = 'We know the Email System is down and are working hard to restore it.
Please do not log tickets at this time.
Sorry for the inconvenience.'

To create more entries, copy the block and paste it below other entries. Rename number at line beginning to the entry number. Note: Entry numbers start with 0. So, in the example above, it is the 2nd entry. If your new entry is the third one, rename all line beginnings to 3.
The date/time field above is treated as Text value and NOT like a date/time field. It will be shown as is and no conversion will happen.