Subtracting time.Duration from time in Go - datetime

Question

I have a time.Time value obtained from time.Now() and I want to get another time which is exactly 1 month ago.
I know subtracting is possible with time.Sub() (which wants another time.Time), but that will result in a time.Duration and I need it the other way around.

Answer 1

In response to Thomas Browne's comment, because lnmx's answer only works for subtracting a date, here is a modification of his code that works for subtracting time from a time.Time type.
package main
import (
"fmt"
"time"
)
func main() {
now := time.Now()
fmt.Println("now:", now)
count := 10
then := now.Add(time.Duration(-count) * time.Minute)
// if we had fix number of units to subtract, we can use following line instead fo above 2 lines. It does type convertion automatically.
// then := now.Add(-10 * time.Minute)
fmt.Println("10 minutes ago:", then)
}
Produces:
now: 2009-11-10 23:00:00 +0000 UTC
10 minutes ago: 2009-11-10 22:50:00 +0000 UTC
Not to mention, you can also use time.Hour or time.Second instead of time.Minute as per your needs.
Playground: https://play.golang.org/p/DzzH4SA3izp

Answer 2

Try AddDate:
package main
import (
"fmt"
"time"
)
func main() {
now := time.Now()
fmt.Println("now:", now)
then := now.AddDate(0, -1, 0)
fmt.Println("then:", then)
}
Produces:
now: 2009-11-10 23:00:00 +0000 UTC
then: 2009-10-10 23:00:00 +0000 UTC
Playground: http://play.golang.org/p/QChq02kisT

Answer 3

You can negate a time.Duration:
then := now.Add(- dur)
You can even compare a time.Duration against 0:
if dur > 0 {
dur = - dur
}
then := now.Add(dur)
You can see a working example at http://play.golang.org/p/ml7svlL4eW

Answer 4

There's time.ParseDuration which will happily accept negative durations, as per manual. Otherwise put, there's no need to negate a duration where you can get an exact duration in the first place.
E.g. when you need to substract an hour and a half, you can do that like so:
package main
import (
"fmt"
"time"
)
func main() {
now := time.Now()
fmt.Println("now:", now)
duration, _ := time.ParseDuration("-1.5h")
then := now.Add(duration)
fmt.Println("then:", then)
}
https://play.golang.org/p/63p-T9uFcZo