21
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xls/date.go
2015-09-30 11:17:25 +08:00

99 lines
3.1 KiB
Go

package xls
import (
"math"
"time"
)
const MJD_0 float64 = 2400000.5
const MJD_JD2000 float64 = 51544.5
func shiftJulianToNoon(julianDays, julianFraction float64) (float64, float64) {
switch {
case -0.5 < julianFraction && julianFraction < 0.5:
julianFraction += 0.5
case julianFraction >= 0.5:
julianDays += 1
julianFraction -= 0.5
case julianFraction <= -0.5:
julianDays -= 1
julianFraction += 1.5
}
return julianDays, julianFraction
}
// Return the integer values for hour, minutes, seconds and
// nanoseconds that comprised a given fraction of a day.
func fractionOfADay(fraction float64) (hours, minutes, seconds, nanoseconds int) {
f := 5184000000000000 * fraction
nanoseconds = int(math.Mod(f, 1000000000))
f = f / 1000000000
seconds = int(math.Mod(f, 60))
f = f / 3600
minutes = int(math.Mod(f, 60))
f = f / 60
hours = int(f)
return hours, minutes, seconds, nanoseconds
}
func julianDateToGregorianTime(part1, part2 float64) time.Time {
part1I, part1F := math.Modf(part1)
part2I, part2F := math.Modf(part2)
julianDays := part1I + part2I
julianFraction := part1F + part2F
julianDays, julianFraction = shiftJulianToNoon(julianDays, julianFraction)
day, month, year := doTheFliegelAndVanFlandernAlgorithm(int(julianDays))
hours, minutes, seconds, nanoseconds := fractionOfADay(julianFraction)
return time.Date(year, time.Month(month), day, hours, minutes, seconds, nanoseconds, time.UTC)
}
// By this point generations of programmers have repeated the
// algorithm sent to the editor of "Communications of the ACM" in 1968
// (published in CACM, volume 11, number 10, October 1968, p.657).
// None of those programmers seems to have found it necessary to
// explain the constants or variable names set out by Henry F. Fliegel
// and Thomas C. Van Flandern. Maybe one day I'll buy that jounal and
// expand an explanation here - that day is not today.
func doTheFliegelAndVanFlandernAlgorithm(jd int) (day, month, year int) {
l := jd + 68569
n := (4 * l) / 146097
l = l - (146097*n+3)/4
i := (4000 * (l + 1)) / 1461001
l = l - (1461*i)/4 + 31
j := (80 * l) / 2447
d := l - (2447*j)/80
l = j / 11
m := j + 2 - (12 * l)
y := 100*(n-49) + i + l
return d, m, y
}
// Convert an excelTime representation (stored as a floating point number) to a time.Time.
func timeFromExcelTime(excelTime float64, date1904 bool) time.Time {
var date time.Time
var intPart int64 = int64(excelTime)
// Excel uses Julian dates prior to March 1st 1900, and
// Gregorian thereafter.
if intPart <= 61 {
const OFFSET1900 = 15018.0
const OFFSET1904 = 16480.0
var date time.Time
if date1904 {
date = julianDateToGregorianTime(MJD_0+OFFSET1904, excelTime)
} else {
date = julianDateToGregorianTime(MJD_0+OFFSET1900, excelTime)
}
return date
}
var floatPart float64 = excelTime - float64(intPart)
var dayNanoSeconds float64 = 24 * 60 * 60 * 1000 * 1000 * 1000
if date1904 {
date = time.Date(1904, 1, 1, 0, 0, 0, 0, time.UTC)
} else {
date = time.Date(1899, 12, 30, 0, 0, 0, 0, time.UTC)
}
durationDays := time.Duration(intPart) * time.Hour * 24
durationPart := time.Duration(dayNanoSeconds * floatPart)
return date.Add(durationDays).Add(durationPart)
}