Fix detection of tooltip fallback format

Use `adapter.startOf(ts, 'day') !== ts` instead of introducing a new adapter API to detect when the timestamp is midnight in the current timezone.

Author: simonbrunel

src/scales/scale.time.js

@@ -417,7 +417,7 @@ function determineLabelFormat(timestamps) {
 		if (ts % INTERVALS.second.size !== 0) {
 			return presets.full;
 		}
-		if (ts % INTERVALS.day.size !== 0) {
+		if (!hasTime && adapter.startOf(ts, 'day') !== ts) {
 			hasTime = true;
 		}
 	}

test/specs/scale.time.tests.js

@@ -662,8 +662,8 @@ describe('Time scale tests', function() {
 				datasets: [{
 					xAxisID: 'xScale0',
 					data: [
-						{t: +new Date('2018-01-08 00:00:00Z'), y: 10},
-						{t: +new Date('2018-01-09 00:00:00Z'), y: 3}
+						{t: +new Date('2018-01-08 00:00:00'), y: 10},
+						{t: +new Date('2018-01-09 00:00:00'), y: 3}
 					]
 				}],
 			},