Restore date 32bit compatibility, be more verbose

Do not use large int64 constants that are not available in GOARCH=386
Fix #239

date.go

@@ -17,12 +17,13 @@ func timeToUTCTime(t time.Time) time.Time {
 func timeToExcelTime(t time.Time) float64 {
 	// TODO in future this should probably also handle date1904 and like TimeFromExcelTime
 	var excelTime float64
+	var deltaDays int64
 	excelTime = 0
+	deltaDays = 290 * 364
 	// check if UnixNano would be out of int64 range
-	for t.Unix() > 9223372036 {
+	for t.Unix() > deltaDays*24*60*60 {
 		// reduce by aprox. 290 years, which is max for int64 nanoseconds
-		deltaDays := 290 * 364
-		delta := time.Duration(deltaDays * 8.64e13)
+		delta := time.Duration(deltaDays) * 24 * time.Hour
 		excelTime = excelTime + float64(deltaDays)
 		t = t.Add(-delta)
 	}