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42 lines
1.5 KiB
42 lines
1.5 KiB
package xlsx
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import (
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"fmt"
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"math"
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"strconv"
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"time"
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)
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// nanoSecondsPerDay defines a constant value of the number of nanoseconds in a day.
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const nanoSecondsPerDay = 24 * 60 * 60 * 1000 * 1000 * 1000
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// excelEpoch specifies the epoch of all excel dates. Dates are internally represented
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// as relative to this value.
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var excelEpoch = time.Date(1899, 12, 30, 0, 0, 0, 0, time.UTC)
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// convertExcelDateToDateString takes an excel numeric representation of a date, and
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// converts it to a human-readable RFC3339 or ISO formatted string.
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//
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// Excel dates are stored within excel as a signed floating point number.
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// The integer portion determines the number of days ahead of 30/12/1899 the date is.
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// The portion after the decimal point represents the proportion through the day.
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// For example, 6am would be 1/4 of the way through a 24hr day, so it is stored as 0.25.
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func convertExcelDateToDateString(value string) (string, error) {
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floatValue, err := strconv.ParseFloat(value, 64)
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if err != nil {
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return "", fmt.Errorf("unable to parse date float value: %w", err)
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}
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numberOfDays := math.Trunc(floatValue)
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numberOfNanoSeconds := (floatValue - numberOfDays) * nanoSecondsPerDay
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actualTime := excelEpoch.AddDate(0, 0, int(numberOfDays)).Add(time.Duration(numberOfNanoSeconds))
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formatString := time.RFC3339
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if floatValue == numberOfDays {
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// We are dealing with a date, and not a datetime
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formatString = "2006-01-02"
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}
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return actualTime.Format(formatString), nil
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}
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