feat: visible date range

Signed-off-by: Sebastian Krupinski <krupinski01@gmail.com>

tests/js/unit/dateRanges.test.ts

@@ -0,0 +1,64 @@
+import { describe, expect, it } from 'vitest'
+import {
+  daysVisibleRange,
+  monthGridVisibleRange,
+  shiftLocalDays,
+  shiftLocalMonths,
+} from '../../../src/utils/dateRanges'
+
+describe('calendar visible ranges', () => {
+  it('uses a half-open range for a day span', () => {
+    const range = daysVisibleRange(new Date(2026, 5, 15, 13, 45), 7)
+    const start = new Date(range.startMs)
+    const end = new Date(range.endMs)
+
+    expect([start.getFullYear(), start.getMonth(), start.getDate(), start.getHours()])
+      .toEqual([2026, 5, 15, 0])
+    expect([end.getFullYear(), end.getMonth(), end.getDate(), end.getHours()])
+      .toEqual([2026, 5, 22, 0])
+  })
+
+  it('uses calendar-day arithmetic across a daylight-saving boundary', () => {
+    const range = daysVisibleRange(new Date(2026, 2, 8, 12), 1)
+    const start = new Date(range.startMs)
+    const end = new Date(range.endMs)
+
+    expect(start.getHours()).toBe(0)
+    expect(end.getHours()).toBe(0)
+    expect(end.getDate()).toBe(start.getDate() + 1)
+  })
+
+  it('includes every leading and trailing day rendered by a month grid', () => {
+    const range = monthGridVisibleRange(new Date(2026, 5, 15))
+    const start = new Date(range.startMs)
+    const end = new Date(range.endMs)
+
+    expect([start.getFullYear(), start.getMonth(), start.getDate()]).toEqual([2026, 4, 31])
+    expect([end.getFullYear(), end.getMonth(), end.getDate()]).toEqual([2026, 6, 5])
+  })
+
+  it('calculates a leap-year February grid', () => {
+    const range = monthGridVisibleRange(new Date(2024, 1, 29))
+    const start = new Date(range.startMs)
+    const end = new Date(range.endMs)
+
+    expect([start.getFullYear(), start.getMonth(), start.getDate()]).toEqual([2024, 0, 28])
+    expect([end.getFullYear(), end.getMonth(), end.getDate()]).toEqual([2024, 2, 3])
+  })
+})
+
+describe('calendar navigation', () => {
+  it('clamps month navigation instead of skipping a shorter month', () => {
+    const shifted = shiftLocalMonths(new Date(2026, 0, 31, 9), 1)
+
+    expect([shifted.getFullYear(), shifted.getMonth(), shifted.getDate(), shifted.getHours()])
+      .toEqual([2026, 1, 28, 9])
+  })
+
+  it('uses calendar-day navigation', () => {
+    const shifted = shiftLocalDays(new Date(2026, 5, 30, 9), 1)
+
+    expect([shifted.getFullYear(), shifted.getMonth(), shifted.getDate(), shifted.getHours()])
+      .toEqual([2026, 6, 1, 9])
+  })
+})