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/datetime.go

@@ -0,0 +1,57 @@
+// Package datetime picks, shows and counts dates and times.
+package datetime
+
+import (
+	"fmt"
+	"strings"
+	"time"
+
+	"github.com/egoist/mygo/ui"
+
+	"git.zacharyzhang.com/ZacharyZhang-NY/8bitMyGoUI/pixel"
+)
+
+// Event is a timed entry of a day.
+type Event struct {
+	Start  time.Time
+	Length time.Duration
+	Title  string
+}
+
+// monthName is m's short name in the frame's locale.
+func monthName(k pixel.Kit, m time.Month) string {
+	return strings.Fields(k.Msg("months"))[m-1]
+}
+
+// weekdays are the day names, Sunday first.
+func weekdays(k pixel.Kit) []string { return strings.Fields(k.Msg("weekdays")) }
+
+// clock prints d as MM:SS, or H:MM:SS past an hour.
+func clock(d time.Duration) string {
+	s := int(d / time.Second)
+	if s >= 3600 {
+		return fmt.Sprintf("%d:%02d:%02d", s/3600, s/60%60, s%60)
+	}
+	return fmt.Sprintf("%02d:%02d", s/60, s%60)
+}
+
+// sameDay reports whether a and b share a date.
+func sameDay(a, b time.Time) bool {
+	ay, am, ad := a.Date()
+	by, bm, bd := b.Date()
+	return ay == by && am == bm && ad == bd
+}
+
+// onDay moves t to d's date, keeping clock and place.
+func onDay(t, d time.Time) time.Time {
+	y, m, day := d.Date()
+	h, mi, s := t.Clock()
+	return time.Date(y, m, day, h, mi, s, t.Nanosecond(), t.Location())
+}
+
+// today is now; the view rebuilds as the day turns.
+func today(c *ui.Context, k pixel.Kit) time.Time {
+	n := k.Now()
+	k.At(c, time.Date(n.Year(), n.Month(), n.Day()+1, 0, 0, 0, 0, n.Location()))
+	return n
+}