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Diffstat (limited to 'TECH.md')
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@@ -0,0 +1,18 @@ +# Time is a mess +ISO C99 is fairly restrictive, while time in the wild is *wild*. + +> The mktime function converts the broken-down time, expressed as local time, in the +> structure pointed to by timeptr into a calendar time value with the same encoding as +> that of the values returned by the time function. The original values of the tm_wday +> and tm_yday components of the structure are ignored, and the original values of the +> other components are not restricted to the ranges indicated above. On successful +> completion, the values of the tm_wday and tm_yday components of the structure are +> set appropriately, and the other components are set to represent the specified calendar +> time, but with their values forced to the ranges indicated above; the final value of +> tm_mday is not set until tm_mon and tm_year are determined. + +This means that `mktime` must operate on `localtime` output. +The implication (which is accurate) implies that `gmtime(t) == gmtime(mktime(localtime(t)))`. + +As such, the source of truth is `time_t`, which is UTC-only. +When we want to modify a `time_t`, we want to use localtime to perform the modification. |