Bad DST handling

[derdilla]

Daylight savings can cause problems in the time interval stepping:

void main() {
  final start = DateTime(2025, 10, 20);
  
  // Bad code, misses an hour
  print(start.add(Duration(days: 7)));
  
  // Better code
  print(start.copyWith(day: start.day + 7));
}

Output:

2025-10-26 23:00:00.000
2025-10-27 00:00:00.000

We do this here

blood-pressure-monitor-fl/app/lib/model/storage/interval_store.dart

Lines 89 to 117 in 821567b

	start: oldStart.add(Duration(days: directionalStep)),
	end: oldEnd.add(Duration(days: directionalStep)),
	),
	TimeStep.week || TimeStep.last7Days => DateRange(
	start: oldStart.add(Duration(days: directionalStep * 7)),
	end: oldEnd.add(Duration(days: directionalStep * 7)),
	),
	TimeStep.month => DateRange(
	// No fitting Duration: wraps correctly according to doc
	start: oldStart.copyWith(month: oldStart.month + directionalStep),
	end: oldEnd.copyWith(month: oldEnd.month + directionalStep),
	),
	TimeStep.year => DateRange(
	// No fitting Duration: wraps correctly according to doc
	start: oldStart.copyWith(year: oldStart.year + directionalStep),
	end: oldEnd.copyWith(year: oldEnd.year + directionalStep),
	),
	TimeStep.lifetime => DateRange(
	start: DateTime.fromMillisecondsSinceEpoch(1),
	end: DateTime.now().copyWith(hour: 23, minute: 59, second: 59),
	),
	TimeStep.last30Days => DateRange(
	start: oldStart.add(Duration(days: directionalStep * 30)),
	end: oldEnd.add(Duration(days: directionalStep * 30)),
	),
	TimeStep.custom => DateRange(
	start: oldStart.add(oldEnd.difference(oldStart) * directionalStep),
	end: oldEnd.add(oldEnd.difference(oldStart) * directionalStep),
	),

This could be the cause behind some of the impossible to reproduce bugs we had about missing measurements.