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extern int _daylight;
_daylight is used by the time and date functions. It is used to tell the RTL's functions (mktime & localtime) whether they should take daylight saving time into account if it runs into a date that would normally fall into that category. _daylight is initialized from the values specified in the TZ environment variable and is set to 1 if the daylight savings time conversion should be applied. If TZ is not set, the value of _daylight is obtained from the operating system.