Adds the following replaceable to notices:

• {time_start:absolute}
• {time_start:relative}
• {time_end:absolute}
• {time_end:relative}

These tags pull from the date-time criteria options for when to start/top showing a notice.

Absolute times are displayed in the user's timezone.

Relative times are not phrased, nor is there any client-side scripting to implement a live countdown.