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date.go 6.15 KB
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xuri 提交于 2020-06-22 00:14 . Update docs and typo fixed
// Copyright 2016 - 2020 The excelize Authors. All rights reserved. Use of
// this source code is governed by a BSD-style license that can be found in
// the LICENSE file.
//
// Package excelize providing a set of functions that allow you to write to
// and read from XLSX / XLSM / XLTM files. Supports reading and writing
// spreadsheet documents generated by Microsoft Exce™ 2007 and later. Supports
// complex components by high compatibility, and provided streaming API for
// generating or reading data from a worksheet with huge amounts of data. This
// library needs Go version 1.10 or later.
package excelize
import (
"errors"
"math"
"time"
)
const (
dayNanoseconds = 24 * time.Hour
maxDuration = 290 * 364 * dayNanoseconds
)
var (
excelMinTime1900 = time.Date(1899, time.December, 31, 0, 0, 0, 0, time.UTC)
excelBuggyPeriodStart = time.Date(1900, time.March, 1, 0, 0, 0, 0, time.UTC).Add(-time.Nanosecond)
)
// timeToExcelTime provides a function to convert time to Excel time.
func timeToExcelTime(t time.Time) (float64, error) {
// TODO in future this should probably also handle date1904 and like TimeFromExcelTime
// Force user to explicit convet passed value to UTC time.
// Because for example 1900-01-01 00:00:00 +0300 MSK converts to 1900-01-01 00:00:00 +0230 LMT
// probably due to daylight saving.
if t.Location() != time.UTC {
return 0.0, errors.New("only UTC time expected")
}
if t.Before(excelMinTime1900) {
return 0.0, nil
}
tt := t
diff := t.Sub(excelMinTime1900)
result := float64(0)
for diff >= maxDuration {
result += float64(maxDuration / dayNanoseconds)
tt = tt.Add(-maxDuration)
diff = tt.Sub(excelMinTime1900)
}
rem := diff % dayNanoseconds
result += float64(diff-rem)/float64(dayNanoseconds) + float64(rem)/float64(dayNanoseconds)
// Excel dates after 28th February 1900 are actually one day out.
// Excel behaves as though the date 29th February 1900 existed, which it didn't.
// Microsoft intentionally included this bug in Excel so that it would remain compatible with the spreadsheet
// program that had the majority market share at the time; Lotus 1-2-3.
// https://www.myonlinetraininghub.com/excel-date-and-time
if t.After(excelBuggyPeriodStart) {
result += 1.0
}
return result, nil
}
// shiftJulianToNoon provides a function to process julian date to noon.
func shiftJulianToNoon(julianDays, julianFraction float64) (float64, float64) {
switch {
case -0.5 < julianFraction && julianFraction < 0.5:
julianFraction += 0.5
case julianFraction >= 0.5:
julianDays++
julianFraction -= 0.5
case julianFraction <= -0.5:
julianDays--
julianFraction += 1.5
}
return julianDays, julianFraction
}
// fractionOfADay provides a function to return the integer values for hour,
// minutes, seconds and nanoseconds that comprised a given fraction of a day.
// values would round to 1 us.
func fractionOfADay(fraction float64) (hours, minutes, seconds, nanoseconds int) {
const (
c1us = 1e3
c1s = 1e9
c1day = 24 * 60 * 60 * c1s
)
frac := int64(c1day*fraction + c1us/2)
nanoseconds = int((frac%c1s)/c1us) * c1us
frac /= c1s
seconds = int(frac % 60)
frac /= 60
minutes = int(frac % 60)
hours = int(frac / 60)
return
}
// julianDateToGregorianTime provides a function to convert julian date to
// gregorian time.
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)
}
// doTheFliegelAndVanFlandernAlgorithm; 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
}
// timeFromExcelTime provides a function to convert an excelTime
// representation (stored as a floating point number) to a time.Time.
func timeFromExcelTime(excelTime float64, date1904 bool) time.Time {
const MDD int64 = 106750 // Max time.Duration Days, aprox. 290 years
var date time.Time
var intPart = 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
const MJD0 float64 = 2400000.5
var date time.Time
if date1904 {
date = julianDateToGregorianTime(MJD0, excelTime+OFFSET1904)
} else {
date = julianDateToGregorianTime(MJD0, excelTime+OFFSET1900)
}
return date
}
var floatPart = 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)
}
// Duration is limited to aprox. 290 years
for intPart > MDD {
durationDays := time.Duration(MDD) * time.Hour * 24
date = date.Add(durationDays)
intPart = intPart - MDD
}
durationDays := time.Duration(intPart) * time.Hour * 24
durationPart := time.Duration(dayNanoSeconds * floatPart)
return date.Add(durationDays).Add(durationPart)
}
// ExcelDateToTime converts a float-based excel date representation to a time.Time.
func ExcelDateToTime(excelDate float64, use1904Format bool) (time.Time, error) {
if excelDate < 0 {
return time.Time{}, newInvalidExcelDateError(excelDate)
}
return timeFromExcelTime(excelDate, use1904Format), nil
}
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