feat: visible date range

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

src/utils/dateRanges.ts

@@ -0,0 +1,55 @@
+import type { VisibleDateRange } from '../types'
+
+export function startOfLocalDay(date: Date): Date {
+  const start = new Date(date)
+  start.setHours(0, 0, 0, 0)
+  return start
+}
+
+export function shiftLocalDays(date: Date, days: number): Date {
+  const shifted = new Date(date)
+  shifted.setDate(shifted.getDate() + days)
+  return shifted
+}
+
+export function shiftLocalMonths(date: Date, months: number): Date {
+  const shifted = new Date(date)
+  const targetDay = shifted.getDate()
+
+  shifted.setDate(1)
+  shifted.setMonth(shifted.getMonth() + months)
+
+  const lastDayOfTargetMonth = new Date(
+    shifted.getFullYear(),
+    shifted.getMonth() + 1,
+    0,
+  ).getDate()
+  shifted.setDate(Math.min(targetDay, lastDayOfTargetMonth))
+
+  return shifted
+}
+
+export function daysVisibleRange(date: Date, days: number): VisibleDateRange {
+  const start = startOfLocalDay(date)
+  const end = shiftLocalDays(start, Math.max(1, days))
+
+  return {
+    startMs: start.getTime(),
+    endMs: end.getTime(),
+  }
+}
+
+export function monthGridVisibleRange(date: Date): VisibleDateRange {
+  const firstDay = new Date(date.getFullYear(), date.getMonth(), 1)
+  const gridStart = new Date(firstDay)
+  gridStart.setDate(gridStart.getDate() - gridStart.getDay())
+
+  const lastDay = new Date(date.getFullYear(), date.getMonth() + 1, 0)
+  const weeks = Math.ceil((firstDay.getDay() + lastDay.getDate()) / 7)
+  const gridEnd = shiftLocalDays(gridStart, weeks * 7)
+
+  return {
+    startMs: gridStart.getTime(),
+    endMs: gridEnd.getTime(),
+  }
+}