If you write applications for cultures other than your own, you will soon confront the challenges of representing currencies, numbers, dates, and times. Currencies vary in both unit value and notation. Numbers are written differently; for example, Europe and the United States use periods and commas in opposite ways. Often a program must display values in notations customary to both vendor and customer.

Scheduling, a common software function, involves time and calendar calculations. Local versions of the Gregorian calendar use different names for days of the week and months, and different ordering for the components of a date. Time may be represented according to a 12- or 24-hour clock, and further complicated by time zone conventions, like Daylight Saving Time (DST), which vary from place to place, or even year to year.

The Standard C Library provides <locale.h> to accommodate some of these different formats, but it is incomplete. It offers no help for conversion from strings to these types, and almost no help for conversions involving two or more locales. Common time zone facilities, such as those defined in POSIX.1 (see the Bibliography), are similarly limited, usually offering no way to compute wall clock time for other locations, or even for the following year in the same location.