amtool silence duration flag always calculates from current time even when --start time is specified

[ktower]

What did you do?
When creating a new silence in the future using amtool with the --start and --duration flags, the end date/time is calculated from the current time rather than the specified start time. See:

alertmanager/cli/silence_add.go

Line 125 in d57b2dc

endsAt = time.Now().UTC().Add(time.Duration(d))

What did you expect to see?
End date/time for a new silence to be calculated from the time specified by the --start flag.

What did you see instead? Under which circumstances?
End date/time is always calculated from the current time. This is particularly notable when the --start time is more than --duration in the future, such as this example run at approximately 2021-02-24T14:00:00Z:

# amtool silence add alertname=testing --start=2021-02-24T23:00:00Z --duration=30m --comment=testing
amtool: error: silence cannot start after it ends

Environment

• System information:

  Linux 5.4.0-65-generic x86_64

• Amtool version:

amtool, version 0.21.0 (branch: HEAD, revision: 4c6c03e)
build user: root@dee35927357f
build date: 20200617-08:54:02
go version: go1.14.4

(binary provided in unmodified quay.io/prometheus/alertmanager:v0.21.0 container)

Note: While I am not a golang programmer, if the maintenance team deems this to be a valid issue I might be interested enough to learn it so that I could contribute to the project.