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   strftime    ( 3 )

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Возвращаемое значение (Return value)

Provided that the result string, including the terminating null
       byte, does not exceed max bytes, strftime() returns the number of
       bytes (excluding the terminating null byte) placed in the array
       s.  If the length of the result string (including the terminating
       null byte) would exceed max bytes, then strftime() returns 0, and
       the contents of the array are undefined.

       Note that the return value 0 does not necessarily indicate an
       error.  For example, in many locales %p yields an empty string.
       An empty format string will likewise yield an empty string.