fix: use local timezone for daily date bucketing instead of UTC

Timestamps in session files are UTC ISO strings. Several code paths
extracted the date via .slice(0, 10) which gives the UTC date, while
date range filtering uses local-time boundaries. This caused turns
between UTC midnight and local midnight to be bucketed under the wrong
day -- the menubar showed lower today cost than the TUI because those
turns were attributed to tomorrow (UTC) but filtered as today (local).

format.ts already had a localDateString fix; this applies the same
pattern everywhere via dateKey() in day-aggregator.ts.

src/cli.ts

@@ -8,7 +8,7 @@ import { renderStatusBar } from './format.js'
 import { type PeriodData, type ProviderCost } from './menubar-json.js'
 import { buildMenubarPayload } from './menubar-json.js'
 import { addNewDays, getDaysInRange, loadDailyCache, saveDailyCache, withDailyCacheLock } from './daily-cache.js'
-import { aggregateProjectsIntoDays, buildPeriodDataFromDays } from './day-aggregator.js'
+import { aggregateProjectsIntoDays, buildPeriodDataFromDays, dateKey } from './day-aggregator.js'
 import { CATEGORY_LABELS, type DateRange, type ProjectSummary, type TaskCategory } from './types.js'
 import { renderDashboard } from './dashboard.js'
 import { parseDateRangeFlags } from './cli-date.js'
@@ -25,7 +25,7 @@ const MS_PER_DAY = 24 * 60 * 60 * 1000
 const BACKFILL_DAYS = 365
 
 function toDateString(date: Date): string {
-  return date.toISOString().slice(0, 10)
+  return `${date.getFullYear()}-${String(date.getMonth() + 1).padStart(2, '0')}-${String(date.getDate()).padStart(2, '0')}`
 }
 
 function getDateRange(period: string): { range: DateRange; label: string } {
@@ -35,12 +35,12 @@ function getDateRange(period: string): { range: DateRange; label: string } {
   switch (period) {
     case 'today': {
       const start = new Date(now.getFullYear(), now.getMonth(), now.getDate())
-      return { range: { start, end }, label: `Today (${start.toISOString().slice(0, 10)})` }
+      return { range: { start, end }, label: `Today (${toDateString(start)})` }
     }
     case 'yesterday': {
       const start = new Date(now.getFullYear(), now.getMonth(), now.getDate() - 1)
       const yesterdayEnd = new Date(now.getFullYear(), now.getMonth(), now.getDate() - 1, 23, 59, 59, 999)
-      return { range: { start, end: yesterdayEnd }, label: `Yesterday (${start.toISOString().slice(0, 10)})` }
+      return { range: { start, end: yesterdayEnd }, label: `Yesterday (${toDateString(start)})` }
     }
     case 'week': {
       const start = new Date(now.getFullYear(), now.getMonth(), now.getDate() - 7)
@@ -123,7 +123,7 @@ function buildJsonReport(projects: ProjectSummary[], period: string, periodKey:
   for (const sess of sessions) {
     for (const turn of sess.turns) {
       if (!turn.timestamp) { continue }
-      const day = turn.timestamp.slice(0, 10)
+      const day = dateKey(turn.timestamp)
       if (!dailyMap[day]) { dailyMap[day] = { cost: 0, calls: 0 } }
       for (const call of turn.assistantCalls) {
         dailyMap[day].cost += call.costUSD
@@ -204,7 +204,7 @@ function buildJsonReport(projects: ProjectSummary[], period: string, periodKey:
     Object.entries(m).sort(([, a], [, b]) => b - a).map(([name, calls]) => ({ name, calls }))
 
   const topSessions = projects
-    .flatMap(p => p.sessions.map(s => ({ project: p.project, sessionId: s.sessionId, date: s.firstTimestamp?.slice(0, 10) ?? null, cost: convertCost(s.totalCostUSD), calls: s.apiCalls })))
+    .flatMap(p => p.sessions.map(s => ({ project: p.project, sessionId: s.sessionId, date: s.firstTimestamp ? dateKey(s.firstTimestamp) : null, cost: convertCost(s.totalCostUSD), calls: s.apiCalls })))
     .sort((a, b) => b.cost - a.cost)
     .slice(0, 5)
 
@@ -545,7 +545,7 @@ program
       return
     }
 
-    const defaultName = `codeburn-${new Date().toISOString().slice(0, 10)}`
+    const defaultName = `codeburn-${toDateString(new Date())}`
     const outputPath = opts.output ?? `${defaultName}.${opts.format}`
 
     let savedPath: string

src/dashboard.tsx

@@ -9,6 +9,7 @@ import { loadPricing } from './models.js'
 import { getAllProviders } from './providers/index.js'
 import { scanAndDetect, type WasteFinding, type WasteAction, type OptimizeResult } from './optimize.js'
 import { estimateContextBudget, discoverProjectCwd, type ContextBudget } from './context-budget.js'
+import { dateKey } from './day-aggregator.js'
 import { join } from 'path'
 
 type Period = 'today' | 'week' | '30days' | 'month' | 'all'
@@ -195,7 +196,7 @@ function DailyActivity({ projects, days = 14, pw, bw }: { projects: ProjectSumma
     for (const session of project.sessions) {
       for (const turn of session.turns) {
         if (!turn.timestamp) continue
-        const day = turn.timestamp.slice(0, 10)
+        const day = dateKey(turn.timestamp)
         dailyCosts[day] = (dailyCosts[day] ?? 0) + turn.assistantCalls.reduce((s, c) => s + c.costUSD, 0)
         dailyCalls[day] = (dailyCalls[day] ?? 0) + turn.assistantCalls.length
       }

src/day-aggregator.ts

@@ -20,8 +20,9 @@ function emptyEntry(date: string): DailyEntry {
   }
 }
 
-function dateKey(iso: string): string {
-  return iso.slice(0, 10)
+export function dateKey(iso: string): string {
+  const d = new Date(iso)
+  return `${d.getFullYear()}-${String(d.getMonth() + 1).padStart(2, '0')}-${String(d.getDate()).padStart(2, '0')}`
 }
 
 export function aggregateProjectsIntoDays(projects: ProjectSummary[]): DailyEntry[] {

src/export.ts

@@ -3,6 +3,7 @@ import { dirname, join, resolve } from 'path'
 
 import { CATEGORY_LABELS, type ProjectSummary, type TaskCategory } from './types.js'
 import { getCurrency, convertCost } from './currency.js'
+import { dateKey } from './day-aggregator.js'
 
 function escCsv(s: string): string {
   const sanitized = /^[=+\-@]/.test(s) ? `'${s}` : s
@@ -48,7 +49,7 @@ function buildDailyRows(projects: ProjectSummary[], period: string): Row[] {
     for (const session of project.sessions) {
       for (const turn of session.turns) {
         if (!turn.timestamp) continue
-        const day = turn.timestamp.slice(0, 10)
+        const day = dateKey(turn.timestamp)
         if (!daily[day]) {
           daily[day] = { cost: 0, calls: 0, input: 0, output: 0, cacheRead: 0, cacheWrite: 0, sessions: new Set() }
         }