feat(tools/when): simple time-conversion tool

I often need unix timestamps, or have unix timestamps, or need timestamps at
certain relative offsets etc.

This adds a tool called `when` which can do all of this. It has basically no
user interface, you just call it with a time query and it does it if it
understands what you meant. This will get smarter over time as I find more uses.

Example:

    ~> when yesterday 5PM
    Local: Mon 06 May 2024 at 17:00:00 MSK
    UTC:   2024-05-06T14:00:00Z
    UNIX:  1715004000

It supports all kinds of queries already, see the usage for details.

Change-Id: I694ffef7608586acfb1ff8010ac0fac4d9951e2e
Reviewed-on: https://cl.tvl.fyi/c/depot/+/11598
Tested-by: BuildkiteCI
Autosubmit: tazjin <tazjin@tvl.su>
Reviewed-by: tazjin <tazjin@tvl.su>

tools/when/default.nix

@@ -0,0 +1,6 @@
+{ depot, ... }:
+
+depot.nix.buildGo.program {
+  name = "when";
+  srcs = [ ./when.go ];
+}

tools/when/when.go

@@ -0,0 +1,141 @@
+package main
+
+import (
+	"fmt"
+	"os"
+	"strconv"
+	"time"
+)
+
+const usage = `usage: when <time>
+
+This program converts the given time into various formats (currently a local
+timestamp, UTC timestamp, and UNIX epoch). It tries to guess what the input is.
+
+Some valid queries:
+
+  2024-01-05
+  1715079241
+  tomorrow 5PM
+  -22h
+  -7h10m
+
+For now a single timestamp and a single duration (which is added either to the
+current time, or the given time) is supported.`
+
+func printTime(t time.Time) {
+	fmt.Println("Local:", t.Format("Mon 02 January 2006 at 15:04:05 MST"))
+	fmt.Println("UTC:  ", t.UTC().Format(time.RFC3339))
+	fmt.Println("UNIX: ", t.Unix())
+}
+
+func setTime(this time.Time, that time.Time) time.Time {
+	return time.Date(
+		this.Year(),
+		this.Month(),
+		this.Day(),
+		that.Hour(),
+		that.Minute(),
+		that.Second(),
+		0,
+		this.Location(),
+	)
+}
+
+func parseTime(input string) (time.Time, error) {
+	// try unix times
+	if i, err := strconv.ParseInt(input, 10, 64); err == nil {
+		if i < 9999999999 {
+			return time.Unix(i, 0), nil
+		}
+		if i < 9999999999999 {
+			return time.UnixMilli(i), nil
+		}
+	}
+
+	// try simple date/time formats
+	if t, err := time.Parse(time.DateOnly, input); err == nil {
+		return t, nil
+	}
+
+	if t, err := time.Parse(time.Kitchen, input); err == nil {
+		now := time.Now()
+		return setTime(now, t), nil
+	}
+
+	if t, err := time.Parse(time.TimeOnly, input); err == nil {
+		now := time.Now()
+		return setTime(now, t), nil
+	}
+
+	if t, err := time.Parse("15:04", input); err == nil {
+		now := time.Now()
+		return setTime(now, t), nil
+	}
+
+	if t, err := time.Parse("3PM", input); err == nil {
+		now := time.Now()
+		return setTime(now, t), nil
+	}
+
+	return time.Time{}, fmt.Errorf("could not parse time: %q", input)
+}
+
+func parseDuration(input string) (time.Duration, error) {
+	// some simple rewriting
+	switch input {
+	case "yesterday":
+		input = "-24h"
+	case "tomorrow":
+		input = "24h"
+	case "today", "now":
+		return time.Duration(0), nil
+	}
+
+	// TODO: days, months, weeks, ...
+	return time.ParseDuration(input)
+}
+
+func main() {
+	if len(os.Args) < 2 {
+		fmt.Fprintln(os.Stderr, usage)
+		os.Exit(1)
+	}
+
+	var d time.Duration
+	var t time.Time
+	var err error
+	var haveTime, haveDuration bool
+
+	for _, arg := range os.Args[1:] {
+		if !haveTime {
+			if t, err = parseTime(arg); err == nil {
+				haveTime = true
+				continue
+			}
+		}
+
+		if !haveDuration {
+			if d, err = parseDuration(arg); err == nil {
+				haveDuration = true
+				continue
+			}
+		}
+	}
+
+	if err != nil {
+		fmt.Fprintln(os.Stderr, "Not sure what you want, try another time.")
+		os.Exit(1)
+	}
+
+	if haveTime && haveDuration {
+		printTime(t.Add(d))
+	} else if haveTime {
+		printTime(t)
+	} else if haveDuration {
+		printTime(time.Now().Add(d))
+	} else {
+		fmt.Fprintln(os.Stderr, "Not sure what you want, try another time.")
+		os.Exit(1)
+	}
+}