build,ui: add build timestamp

together with the build semver, this should provide stable info for
ndg software version running on a given device.

build.zig

@@ -14,6 +14,7 @@ pub fn build(b: *std.build.Builder) void {
 
     const buildopts = b.addOptions();
     buildopts.addOption(DriverTarget, "driver", drv);
+    buildopts.addOption(u64, "build_timestamp", @intCast(u64, std.time.timestamp()));
     const semver_step = VersionStep.create(b, buildopts, inver);
 
     // gui build

src/ui/ui.zig

@@ -61,7 +61,24 @@ export fn nm_create_info_panel(parent: *lvgl.LvObj) c_int {
 }
 
 fn createInfoPanel(parent: *lvgl.LvObj) !void {
+    parent.flexFlow(.column);
+    parent.flexAlign(.start, .start, .start);
+
     var buf: [100]u8 = undefined;
-    const s = try std.fmt.bufPrintZ(&buf, "ndg version: {any}", .{buildopts.semver});
+    const sver = try std.fmt.bufPrintZ(&buf, "nakamochi software version: {any}", .{buildopts.semver});
-    _ = try lvgl.createLabel(parent, s, .{ .long_mode = .wrap, .pos = .none });
+    _ = try lvgl.createLabel(parent, sver, .{ .long_mode = .wrap, .pos = .none });
+
+    const epoch: std.time.epoch.EpochSeconds = .{ .secs = buildopts.build_timestamp };
+    const daysec = epoch.getDaySeconds();
+    const yearday = epoch.getEpochDay().calculateYearDay();
+    const monthday = yearday.calculateMonthDay();
+    const ts = try std.fmt.bufPrintZ(&buf, "build timestamp: {d}-{d:0>2}-{d:0>2} {d:0>2}:{d:0>2}:{d:0>2} UTC", .{
+        yearday.year,
+        monthday.month.numeric(),
+        monthday.day_index + 1,
+        daysec.getHoursIntoDay(),
+        daysec.getMinutesIntoHour(),
+        daysec.getSecondsIntoMinute(),
+    });
+    _ = try lvgl.createLabel(parent, ts, .{ .long_mode = .wrap, .pos = .none });
 }