The Timezone Calculator modules provides a block, which can be placed at any region.
The Bock contains a link, on click of that link, will provide a form where the user will enter the date and time for which he wish to see the dates in different time zones.
User can also have to select the timezones, which he wants to see as the result.

The Timezone Calculator is very useful, where we can see a selected date in different timezones without any manual calculations.