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Fix j2day() to behave sanely for negative Julian dates.

Somebody had apparently once figured that casting to unsigned int wouldproduce the right output for negative inputs, but that would only betrue if 2^32 were a multiple of 7, which of course it ain't. We needto use a signed division and then correct the sign of the remainder.AFAICT, the only case where this would arise currently is when doingISO-week calculations for dates in 4714BC, where we'd compute anegative Julian date representing 4714-01-04BC and then do somearithmetic with it. Since we don't even really document support forsuch dates, this is not of much consequence. But we may as wellget it right.Per report from Vitaly Burovoy.

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src/backend/utils/adt
- datetime.c

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`‎src/backend/utils/adt/datetime.c`

Lines changed: 6 additions & 7 deletions

Original file line number	Diff line number	Diff line change
`@@ -355,14 +355,13 @@ j2date(int jd, int year, int month, int *day)`
`355`	`355`	`int`
`356`	`356`	`j2day(intdate)`
`357`	`357`	`{`
`358`		`-unsignedintday;`
	`358`	`+date+=1;`
	`359`	`+date %=7;`
	`360`	`+/* Cope if division truncates towards zero, as it probably does */`
	`361`	`+if (date<0)`
	`362`	`+date+=7;`
`359`	`363`
`360`		`-day=date;`
`361`		`-`
`362`		`-day+=1;`
`363`		`-day %=7;`
`364`		`-`
`365`		`-return (int)day;`
	`364`	`+returndate;`
`366`	`365`	`}/* j2day() */`
`367`	`366`
`368`	`367`

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Commit5db5146

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`‎src/backend/utils/adt/datetime.c`

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