To get the current time in various cities or time zones, you can use Local Times Around the World, The World Clock or the Time Zone Converter. You can also call anyone there and ask them for the time (if you speak their language). If that doesn't work, try calling the International Operator and asking for the time in the location you need.
For current, historical or future time comparisons use the Time Zone Converter, which covers from 1970 through 2037.
Standard and Daylight Saving Time: Most of the world relies on "Standard Time," which involves slicing the globe vertically into 24 time zones, starting with the longitude line running through Greenwich, England. There are several variations based on Standard Time, notably "Daylight Saving Time."
"Daylight Saving Time" means moving the clock forward one hour starting 2 a.m. on the last Sunday in April and moving it back to Standard time at 2 a.m. on the last Sunday in October. Under the Uniform Time Act (1966), all U.S. jurisdictions must follow these dates if they use Daylight Savings Time. However Arizona, Hawaii, Eastern Indiana, Puerto Rico, the U.S. Virgin Islands and American Samoa don't use Daylight Saving Time at all.
Most of Canada and Mexico use Daylight Saving Time too. Other countries use variations. For more information, see The World Almanac or another reference source.
Religious Time: Kashrut.com posts a calculator for figuring the time of day under Jewish law. There is also a site for figuring Muslim prayer times.
Days: Use the resources in the "Calendars" entry to figure the number of days in a year or to calculate the number of days from a date or between two dates.