package xlsx import ( "math" "time" ) const ( MJD_0 float64 = 2400000.5 MJD_JD2000 float64 = 51544.5 secondsInADay = float64((24*time.Hour)/time.Second) nanosInADay = float64((24*time.Hour)/time.Nanosecond) ) var ( timeLocationUTC, _ = time.LoadLocation("UTC") unixEpoc = time.Date(1970, time.January, 1, 0, 0, 0, 0, time.UTC) // In 1900 mode, Excel takes dates in floating point numbers of days starting with Jan 1 1900. // The days are not zero indexed, so Jan 1 1900 would be 1. // Except that Excel pretends that Feb 29, 1900 occurred to be compatible with a bug in Lotus 123. // So, this constant uses Dec 30, 1899 instead of Jan 1, 1900, so the diff will be correct. // http://www.cpearson.com/excel/datetime.htm excel1900Epoc = time.Date(1899, time.December, 30, 0, 0, 0, 0, time.UTC) excel1904Epoc = time.Date(1904, time.January, 1, 0, 0, 0, 0, time.UTC) // Days between epocs, including both off by one errors for 1900. daysBetween1970And1900 = float64(unixEpoc.Sub(excel1900Epoc)/(24 * time.Hour)) daysBetween1970And1904 = float64(unixEpoc.Sub(excel1904Epoc)/(24 * time.Hour)) ) func TimeToUTCTime(t time.Time) time.Time { return time.Date(t.Year(), t.Month(), t.Day(), t.Hour(), t.Minute(), t.Second(), t.Nanosecond(), timeLocationUTC) } 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. // 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 } 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 wholeDaysPart = int(excelTime) // Excel uses Julian dates prior to March 1st 1900, and // Gregorian thereafter. if wholeDaysPart <= 61 { const OFFSET1900 = 15018.0 const OFFSET1904 = 16480.0 var date time.Time if date1904 { date = julianDateToGregorianTime(MJD_0, excelTime+OFFSET1904) } else { date = julianDateToGregorianTime(MJD_0, excelTime+OFFSET1900) } return date } var floatPart = excelTime - float64(wholeDaysPart) if date1904 { date = excel1904Epoc } else { date = excel1900Epoc } durationPart := time.Duration(nanosInADay * floatPart) return date.AddDate(0,0, wholeDaysPart).Add(durationPart) } // TimeToExcelTime will convert a time.Time into Excel's float representation, in either 1900 or 1904 // mode. If you don't know which to use, set date1904 to false. // TODO should this should handle Julian dates? func TimeToExcelTime(t time.Time, date1904 bool) float64 { // Get the number of days since the unix epoc daysSinceUnixEpoc := float64(t.Unix())/secondsInADay // Get the number of nanoseconds in days since Unix() is in seconds. nanosPart := float64(t.Nanosecond())/nanosInADay // Add both together plus the number of days difference between unix and Excel epocs. var offsetDays float64 if date1904 { offsetDays = daysBetween1970And1904 } else { offsetDays = daysBetween1970And1900 } daysSinceExcelEpoc := daysSinceUnixEpoc + offsetDays + nanosPart return daysSinceExcelEpoc }