You must enter the target date and time and the Timer Countdown plugin will automatically calculate the remaining time and display it on the pages of your website that has the [timer-countdown] shortcode.
You will be able to display the remaining time in years, months, days, hours, minutes, and seconds; or any configuration you decide.
The remaining time will be calculated taking into account the time of your WordPress installation and not the local time of each connected user. In that way when the count reaches zero, it will reach zero for all users at the same time, regardless of where in the world they are.