Add datetime components 196-215; Kit.At; inert stepper arrows

Calendars, date and time inputs, pickers, clocks, timers, a sun dial,
a season wheel, an agenda and a day schedule. Kit.At rebuilds at a
moment (relative times, midnight). Stepper and NumberInput arrows stop
under Inert. DateInput parks a popup focus as its own.

datetime/relativetime.go

@@ -0,0 +1,56 @@
+package datetime
+
+import (
+	"fmt"
+	"time"
+
+	"github.com/egoist/mygo/ui"
+
+	"git.zacharyzhang.com/ZacharyZhang-NY/8bitMyGoUI/pixel"
+)
+
+type RelativeTimeOptions struct {
+	// Slot inks the text; zero means Faint.
+	Slot pixel.Slot
+}
+
+// RelativeTime says how long since or until t, each minute.
+func RelativeTime(c *ui.Context, t time.Time, o RelativeTimeOptions) *ui.Element {
+	k := pixel.K(c)
+	ink := o.Slot
+	if ink == 0 {
+		ink = pixel.Faint
+	}
+	// Minutes since t; ahead of it, rounded up and negative.
+	d, m := k.Now().Sub(t), 0
+	if d >= 0 {
+		m = int(d / time.Minute)
+	} else {
+		m = -int((-d + time.Minute - 1) / time.Minute)
+	}
+	k.At(c, t.Add(time.Duration(m+1)*time.Minute))
+	return pixel.Text(c, words(k, m), pixel.TextOptions{Slot: ink})
+}
+
+// words says m minutes: ago when positive, ahead when negative.
+func words(k pixel.Kit, m int) string {
+	n := max(m, -m)
+	h, days := n/60, n/(24*60)
+	pick := func(past, future string) string {
+		if m > 0 {
+			return past
+		}
+		return future
+	}
+	switch {
+	case n == 0:
+		return k.Msg("just_now")
+	case h == 0:
+		return fmt.Sprintf(k.Msg(pick("min_ago", "in_min")), n)
+	case days == 0:
+		return fmt.Sprintf(k.Msg(pick("hours_ago", "in_hours")), h)
+	case days == 1:
+		return k.Msg(pick("yesterday", "tomorrow"))
+	}
+	return fmt.Sprintf(k.Msg(pick("days_ago", "in_days")), days)
+}