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65 lines
2.2 KiB
Plaintext
65 lines
2.2 KiB
Plaintext
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This file contains description of our my_timegm() function in date.c. It was
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posted as a mail to links-list by Stephane Chazelas, and I thought it may be
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interesting for someone, so I decided to include it in the ELinks distribution.
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Un explanation for it as one (me to start with) may wonder why
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this works.
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We first change of calendar. To make things easy, let's say
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that 0/0/0 0:0:0 in our new calendar is the Marsh 1st 1968, so
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just after a february 29th as 1968 was a leap year.
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if y/m/d h:min:s is time in our calendar
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and
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Y/M/D h:min:s in the new calendar
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M = m - 1 - 2 (+ 12 if m < 3)
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Y = y - 68 (-1 if m < 3)
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D = d - 1
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at Y/0/0 0:0:0, there has been Y / 4 leap years in the past, so
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(int) 365 * Y + Y / 4 days have past.
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at Y/M/0 0:0:0, lets find how many days have past since Y/0/0:
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|Mar Feb
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M| 0 1 2 3 4 5 6 7 8 9 10 11
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-------------------+-----------------------------------------------
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days in that month|31 30 31 30 31 31 30 31 30 31 31 28 or 29
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-------------------+-----------------------------------------------
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x = days at Y/M/0| 0 31 61 92 122 153 184 214 245 275 306 337
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-------------------+-----------------------------------------------
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first approximation|
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y = 30 * M| 0 30 60 90 120 150 180 210 240 270 300 330
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-------------------+-----------------------------------------------
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x - y| 0 1 1 2 2 3 4 4 5 5 6 7
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-------------------+-----------------------------------------------
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(M + 4) * 3 / 5 - 2| 0 1 1 2 2 3 4 4 5 5 6 7
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-------------------+-----------------------------------------------
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x - y = (M + 4) * 3 / 5 - 2
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x = 30 * M + (M + 4) * 3 / 5 - 2
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x = (153 * M + 2) / 5
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So at Y/M/D 0:0:0,
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Y * 1461 / 4 + (153 * M + 2) / 5 + D days have past since
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the 1st of March of 1968
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1st of March of 1968 was 671 days before 1970 Jan 1st (year 0
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for unix time())
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So
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t = s + 60 * (min + 60 * (h + 24 * (Y * 1461 / 4 + (153 * M + 2) / 5 + D - 671)))
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t = s + 60 * (min + 60 * (h + 24 * (Y * 1461 / 4 + (153 * M + 2) / 5 + d - 672)))
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This shouldn't work past 2100/02/28 23:59:59 as 2100 is not a leap year.
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--
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St<EFBFBD>phane
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