RTCs for the RasPi that don't use the GPIO pins but a USB jack are apparently a thing. Battery backed.
Software for using those RTCs: https://github.com/sbcshop/USB-RTC
WorldTimeAPI is a simple web service which returns the local-time for a given timezone as either JSON or plain-text. Some additional information is provided, such as whether that timezone is currently in Daylight Savings Time, when DST starts and ends, the UTC offset, etc. Does not require an account or API key. Free.
A site that converts UNIX style time_t timestamps into human readable time/date stamps.
A website with lots of different conversion utilities. Plug in the values, get the answers, and get on with your life.
TimeZoneDB provides free time zone database for cities of the world. The database is licensed under Creative Commons Attribution 3.0 License. It contains countries name, time zones, abbreviation, GMT offset, and Daylight Saving Time (DST). The data is available in CSV and SQL format. You can download and implement into your projects for free.
API access also available to query time zone information. You can get time zone data by latitude & longitude, or zone name. You need to register a API key to access the API.
REST Services that will return current date/time in JSON for any registered time zone. Seems pretty straightforward and easy to use.
The homepage of Terrence McKenna (RIP).
An on-line web application that shows you what time it is in every other time zone around the world relative to the current time. Handy if you're trying to figure out if you should ping someone at a particular hour.