fix(events): seven-date default period and picker from today (WealthLedger/frontend#63)

src/shared/lib/calendar.ts

@@ -9,8 +9,7 @@ function isValidPeriod(from: null | string, to: null | string) {
 	);
 }
 
-const DAY_MS = 86_400_000;
-const DEFAULT_PERIOD_DAYS = 6;
+const DEFAULT_PERIOD_LAST_INDEX = 6;
 
 const moscowFormatter = new Intl.DateTimeFormat("en-CA", {
 	day: "2-digit",
@@ -25,12 +24,17 @@ function compareDateDesc(a: null | string, b: null | string) {
 	return (b ?? "").localeCompare(a ?? "");
 }
 
-// Last 7 days including today. An empty period always means this range.
+// 7 calendar dates starting with today (Moscow calendar arithmetic).
 function defaultPeriod(): [string, string] {
-	const to = new Date();
-	const from = new Date(to.getTime() - DEFAULT_PERIOD_DAYS * DAY_MS);
+	const from = moscowDate(new Date());
 
-	return [moscowDate(from), moscowDate(to)];
+	const [year = 1970, month = 0, day = 1] = from.split("-").map(Number);
+	const noon = 12;
+	const lastDay = day + DEFAULT_PERIOD_LAST_INDEX;
+	const end = new Date(Date.UTC(year, month - 1, lastDay, noon));
+	const to = moscowDate(end);
+
+	return [from, to];
 }
 
 function moscowDate(value: Date): string {