如何获得最后一个季度

Here below is my code to get the last complete quarter:

package main

import (
    "fmt"
    "time"
)

func main() {
    layout := "2006-01-02T15:04:05.000Z"
    str := "2017-11-30T12:00:00.000Z"
    now, _ := time.Parse(layout, str)

    endDate := now.AddDate(0, 0, 0-now.Day())
    startDate := endDate.AddDate(0, -3, 0) // startDate is wrong: 2017-07-31
    // the following statement is needed to fix startDate
    if endDate.Month()-startDate.Month() == 3 {
        startDate = startDate.AddDate(0, 0, 1) // now startDate is correct: 2017-08-01
    }

    fmt.Printf("Start date: %v
", startDate.Format("2006-01-02"))
    fmt.Printf("End date: %v
", endDate.Format("2006-01-02"))
}

playground

Is there a better way to get the correct start date?

For instance, the last startDate = startDate.AddDate(0, 0, 1) statement has to be omitted if I want to get the last semester:

endDate := now.AddDate(0, 0, 0-now.Day())
startDate := endDate.AddDate(0, -6, 0) // startDate is correct: 2017-05-01

Why is there this difference?

Package time

import "time"

func Date

func Date(year int, month Month, day, hour, min, sec, nsec int, loc *Location) Time

Date returns the Time corresponding to

yyyy-mm-dd hh:mm:ss + nsec nanoseconds

in the appropriate zone for that time in the given location.

The month, day, hour, min, sec, and nsec values may be outside their usual ranges and will be normalized during the conversion. For example, October 32 converts to November 1.

For example, using normalization to get the last complete period (for example, quarter or semester):

package main

import (
    "fmt"
    "time"
)

func lastPeriod(t time.Time, period time.Month) (start, end time.Time) {
    y, m, _ := t.Date()
    loc := t.Location()
    start = time.Date(y, m-period, 1, 0, 0, 0, 0, loc)
    end = time.Date(y, m, 1, 0, 0, 0, -1, loc)
    return start, end
}

func main() {
    layout := "2006-01-02T15:04:05.000Z"
    str := "2017-11-30T12:00:00.000Z"
    now, err := time.Parse(layout, str)
    if err != nil {
        fmt.Println(err)
        return
    }
    const (
        quarter  = 3
        semester = 6
    )
    fmt.Println("Quarter:")
    start, end := lastPeriod(now, quarter)
    fmt.Printf("Base date:  %v
", now.Format("2006-01-02"))
    fmt.Printf("Start date: %v
", start.Format("2006-01-02"))
    fmt.Printf("End date:   %v
", end.Format("2006-01-02"))
    fmt.Println("Semester:")
    start, end = lastPeriod(now, semester)
    fmt.Printf("Base date:  %v
", now.Format("2006-01-02"))
    fmt.Printf("Start date: %v
", start.Format("2006-01-02"))
    fmt.Printf("End date:   %v
", end.Format("2006-01-02"))
}

Playground: https://play.golang.org/p/0t4exjVgr-

Output:

Quarter:
Base date:  2017-11-30
Start date: 2017-08-01
End date:   2017-10-31
Semester:
Base date:  2017-11-30
Start date: 2017-05-01
End date:   2017-10-31