diff --git a/src/cli.ts b/src/cli.ts
index 494e313..6090495 100644
--- a/src/cli.ts
+++ b/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
diff --git a/src/dashboard.tsx b/src/dashboard.tsx
index a4e5cde..99163aa 100644
--- a/src/dashboard.tsx
+++ b/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
       }
diff --git a/src/day-aggregator.ts b/src/day-aggregator.ts
index 5030f8d..bc63fa6 100644
--- a/src/day-aggregator.ts
+++ b/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[] {
diff --git a/src/export.ts b/src/export.ts
index 58f250e..2ef75bd 100644
--- a/src/export.ts
+++ b/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() }
         }