cmd/tailscale/cli: fix time in tailscale routecheck (#19956)

When running `tailscale netcheck`, the reported timestamp used to be
in UTC and formatted according to RFC 3339 with a `T` to separate the
date from the time:

	sfllaw@h2co3:~$ tailscale netcheck | head -n3

	Report:
		* Time: 2026-06-01T21:12:32.252620138Z

This is machine-readable time leaking out to the user interface. Times
in normal commands are formatted for humans to read:

	sfllaw@h2co3:~$ date
	Mon 01 Jun 2026 02:39:14 PM PDT
	sfllaw@h2co3:~$ journalctl -t tailscaled | tail -n1
	Jun 01 14:35:21 h2co3 tailscaled[3328921]: wgengine: sending TSMP disco key advertisement to 100.90.144.102
	sfllaw@h2co3:~$ timedatectl show
	Timezone=America/Los_Angeles
	LocalRTC=no
	CanNTP=yes
	NTP=yes
	NTPSynchronized=yes
	TimeUSec=Mon 2026-06-01 14:38:32 PDT
	RTCTimeUSec=Mon 2026-06-01 14:38:32 PDT
	sfllaw@h2co3:~$ uptime --since
	2026-05-15 07:37:45

This PR makes the times printed by the CLI commands consistent:

- For `tailscale routecheck`, it now prints local time as
  `2026-05-15 07:37:45-07:00`.
- For `netlogfmt`, it has always printed local time with a space,
  but now includes the time zone.
- All machine-readable outputs continue to be standard RFC 3339 in
  UTC, i.e. `--format=json`.

As part of a general cleanup, this PR also adds standard common
time.Format layouts as tstime constants.

Fixes #19928

Signed-off-by: Simon Law <sfllaw@tailscale.com>

cmd/netlogfmt/main.go

@@ -45,6 +45,7 @@ import (
 	jsonv2 "github.com/go-json-experiment/json"
 	"github.com/go-json-experiment/json/jsontext"
 	"tailscale.com/tailcfg"
+	"tailscale.com/tstime"
 	"tailscale.com/types/bools"
 	"tailscale.com/types/logid"
 	"tailscale.com/types/netlogtype"
@@ -294,7 +295,7 @@ func printMessage(msg message) {
 		fmt.Printf("NodeID: %s\n", msg.NodeID)
 	}
 	formatTime := func(t time.Time) string {
-		return t.In(time.Local).Format("2006-01-02 15:04:05.000")
+		return t.Local().Format(tstime.DateSpTimeMilliZ)
 	}
 	switch {
 	case !msg.Logged.IsZero():

cmd/stund/depaware.txt

@@ -61,6 +61,7 @@ tailscale.com/cmd/stund dependencies: (generated by github.com/tailscale/depawar
         tailscale.com/net/tsaddr                                     from tailscale.com/tsweb
         tailscale.com/syncs                                          from tailscale.com/metrics+
         tailscale.com/tailcfg                                        from tailscale.com/version+
+        tailscale.com/tstime                                         from tailscale.com/tsweb
         tailscale.com/tsweb                                          from tailscale.com/cmd/stund+
         tailscale.com/tsweb/promvarz                                 from tailscale.com/cmd/stund
         tailscale.com/tsweb/varz                                     from tailscale.com/tsweb+

cmd/tailscale/cli/netcheck.go

@@ -26,6 +26,7 @@ import (
 	"tailscale.com/net/portmapper/portmappertype"
 	"tailscale.com/net/tlsdial"
 	"tailscale.com/tailcfg"
+	"tailscale.com/tstime"
 	"tailscale.com/types/logger"
 	"tailscale.com/util/eventbus"
 	"tailscale.com/util/set"
@@ -175,7 +176,7 @@ func printNetCheckReport(dm *tailcfg.DERPMap, report *netcheck.Report) error {
 	}
 
 	printf("\nReport:\n")
-	printf("\t* Time: %v\n", report.Now.Format(time.RFC3339Nano))
+	printf("\t* Time: %v\n", report.Now.Local().Format(tstime.DateSpTimeNanoZ))
 	printf("\t* UDP: %v\n", report.UDP)
 	if report.GlobalV4.IsValid() {
 		printf("\t* IPv4: yes, %s\n", report.GlobalV4)