Add Formula & Format support

date.go

@@ -5,37 +5,57 @@ import (
 	"time"
 )
 
-const MJD_0 float64 = 2400000.5
-const MJD_JD2000 float64 = 51544.5
+// timeLocationUTC defined the UTC time location.
+var timeLocationUTC, _ = time.LoadLocation("UTC")
 
+// timeToUTCTime provides function to convert time to UTC time.
+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)
+}
+
+// timeToExcelTime provides function to convert time to Excel time.
+func timeToExcelTime(t time.Time) float64 {
+	return float64(t.UnixNano())/8.64e13 + 25569.0
+}
+
+// shiftJulianToNoon provides 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 += 1
+		julianDays++
 		julianFraction -= 0.5
 	case julianFraction <= -0.5:
-		julianDays -= 1
+		julianDays--
 		julianFraction += 1.5
 	}
 	return julianDays, julianFraction
 }
 
-// Return the integer values for hour, minutes, seconds and
-// nanoseconds that comprised a given fraction of a day.
+// fractionOfADay provides 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) {
-	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
+
+	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 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)
@@ -47,13 +67,12 @@ func julianDateToGregorianTime(part1, part2 float64) time.Time {
 	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.
+// 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
@@ -68,24 +87,26 @@ func doTheFliegelAndVanFlandernAlgorithm(jd int) (day, month, year int) {
 	return d, m, y
 }
 
-// Convert an excelTime representation (stored as a floating point number) to a time.Time.
+// timeFromExcelTime provides function to 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.
+	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(MJD_0+OFFSET1904, excelTime)
+			date = julianDateToGregorianTime(MJD0, excelTime+OFFSET1904)
 		} else {
-			date = julianDateToGregorianTime(MJD_0+OFFSET1900, excelTime)
+			date = julianDateToGregorianTime(MJD0, excelTime+OFFSET1900)
 		}
 		return date
 	}
-	var floatPart float64 = excelTime - float64(intPart)
+	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)