Add option to count down by seconds rather than datetime

[psilordOG]

Counting down by a target datetime value is fine except for the fact that the local device date and time is used for comparison. Relying on the local device to have accurate time is unreliable and untrustworthy, especially when absolute control is required. An end user can easily change the date and time on their device, thereby altering when the countdown timer was intended to fire.

Adding an option to count down by a number of seconds (or milliseconds) that is configurable via an attribute would allow the server to control when the countdown ends and the action is fired.