Don't know about other operating systems but on linux (and I think Mac) you can use the date command at the command line:
Want to know what time it will be locally when it is 12 noon in Sydney, Australia next Monday?
date --date='TZ="Australia/Sydney" 12:00 next Mon'
If you know how to write shell scripts you could incorporate this command into one.
date is part of the Gnu coreutils package
Other relevant links:
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