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diff --git a/src/nwc-budget.js b/src/nwc-budget.js
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+++ b/src/nwc-budget.js
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+// An NWC connection's spending budget resets on a calendar boundary — a day,
+// a week or a month — in UTC, so every device of a wallet (and the service
+// worker answering while the app is closed) agrees on which window a spend
+// fell in without a timezone in the picture. The window key is what the
+// connection's spent counter is stamped with: a counter stamped with another
+// key is from a window that has passed.
+
+export const PERIODS = ['day', 'week', 'month'];
+
+const pad = (n) => String(n).padStart(2, '0');
+
+// The week runs Monday to Sunday (ISO), keyed by its Monday.
+function mondayOf(d) {
+  const m = new Date(d);
+  const dow = (m.getUTCDay() + 6) % 7; // Monday = 0
+  m.setUTCDate(m.getUTCDate() - dow);
+  m.setUTCHours(0, 0, 0, 0);
+  return m;
+}
+
+export function budgetWindow(period, ts = Date.now()) {
+  const d = new Date(ts);
+  if (period === 'month') return `${d.getUTCFullYear()}-${pad(d.getUTCMonth() + 1)}`;
+  if (period === 'week') { const m = mondayOf(d); return `W${m.getUTCFullYear()}-${pad(m.getUTCMonth() + 1)}-${pad(m.getUTCDate())}`; }
+  return `${d.getUTCFullYear()}-${pad(d.getUTCMonth() + 1)}-${pad(d.getUTCDate())}`;
+}
+
+// When the current window ends, in unix seconds (NIP-47's renews_at).
+export function budgetRenewsAt(period, ts = Date.now()) {
+  const d = new Date(ts);
+  let end;
+  if (period === 'month') end = Date.UTC(d.getUTCFullYear(), d.getUTCMonth() + 1, 1);
+  else if (period === 'week') { const m = mondayOf(d); end = m.getTime() + 7 * 86_400_000; }
+  else end = Date.UTC(d.getUTCFullYear(), d.getUTCMonth(), d.getUTCDate() + 1);
+  return Math.floor(end / 1000);
+}
+
+// NIP-47 names the period as an adverb.
+export const renewalPeriod = (period) => (period === 'month' ? 'monthly' : period === 'week' ? 'weekly' : 'daily');
+
+// What a connection has spent in the window that is on now: the counter
+// carried on the connection (stamped with its window) plus, for the worker,
+// its own spends the app has not yet folded into that counter.
+export function spentInWindow(conn, extraSpends = [], ts = Date.now()) {
+  const period = conn.period || 'day';
+  const win = budgetWindow(period, ts);
+  let spent = conn.spentDate === win ? (conn.spentToday || 0) : 0;
+  for (const s of extraSpends) {
+    if (s.absorbed || s.connId !== conn.id) continue;
+    if (budgetWindow(period, s.ts) !== win) continue;
+    spent += (s.amountSat || 0) + (s.feeSat || 0);
+  }
+  return { window: win, period, spent, left: Math.max(0, (conn.dailySat || 0) - spent) };
+}