diff --git a/internal/chat/queries/usage.sql b/internal/chat/queries/usage.sql
index 778a6d0..4bade39 100644
--- a/internal/chat/queries/usage.sql
+++ b/internal/chat/queries/usage.sql
@@ -27,7 +27,7 @@ GROUP BY model_id
 ORDER BY cost_usd DESC;
 
 -- name: SummarizeUsageByDay :many
-SELECT date_trunc('day', created_at) AS day,
+SELECT date_trunc('day', created_at)::timestamptz AS day,
        SUM(prompt_tokens)::int     AS prompt_tokens,
        SUM(completion_tokens)::int AS completion_tokens,
        SUM(thinking_tokens)::int   AS thinking_tokens,
@@ -38,7 +38,7 @@ GROUP BY day
 ORDER BY day;
 
 -- name: SummarizeUserDaily :many
-SELECT date_trunc('day', created_at) AS day,
+SELECT date_trunc('day', created_at)::timestamptz AS day,
        SUM(prompt_tokens)::int     AS prompt_tokens,
        SUM(completion_tokens)::int AS completion_tokens,
        SUM(thinking_tokens)::int   AS thinking_tokens,
diff --git a/internal/chat/repository.go b/internal/chat/repository.go
index 7646257..056304a 100644
--- a/internal/chat/repository.go
+++ b/internal/chat/repository.go
@@ -25,7 +25,6 @@ import (
 )
 
 // Postgres epoch: 2000-01-01 00:00:00 UTC
-var pgEpoch = time.Date(2000, 1, 1, 0, 0, 0, 0, time.UTC)
 
 // Repository 是 chat 模块对 PG 的全部依赖。
 // Service 单元测试通过手写 fakeRepo 满足该接口。
@@ -276,17 +275,12 @@ func float64ToNumeric(f float64) pgtype.Numeric {
 	return n
 }
 
-// intervalToTime は date_trunc('day',...) が pgtype.Interval として返ってくる
-// sqlc 型推論ドリフトへの対処。Postgres は timestamptz を binary で
-// 「2000-01-01 エポックからのマイクロ秒」として送るが、pgx が pgtype.Interval
-// へスキャンした場合 Microseconds フィールドに入る。
-// Drift 注記: sqlc v1.31.1 は date_trunc の返り型を誤って interval と推論する。
-func intervalToTime(iv pgtype.Interval) time.Time {
-	if !iv.Valid {
+// timestamptzToTime safely extracts time.Time from pgtype.Timestamptz.
+func timestamptzToTime(ts pgtype.Timestamptz) time.Time {
+	if !ts.Valid {
 		return time.Time{}
 	}
-	// Microseconds はここでは「Postgres epoch からのマイクロ秒数」として解釈する。
-	return pgEpoch.Add(time.Duration(iv.Microseconds) * time.Microsecond)
+	return ts.Time
 }
 
 // ===== PG エラー変換 =====
@@ -462,10 +456,7 @@ func modelUsageRowFromRow(r chatsqlc.SummarizeUsageByModelRow) ModelUsageRow {
 
 func dayUsageRowFromRow(r chatsqlc.SummarizeUsageByDayRow) DayUsageRow {
 	return DayUsageRow{
-		// Drift: sqlc 把 date_trunc('day', timestamptz) 推断为 pgtype.Interval。
-		// pgx 在 binary 模式下把 timestamptz 值的 "Postgres epoch + microseconds"
-		// 送入 Interval.Microseconds 字段。用 pgEpoch + Duration 还原。
-		Day:              intervalToTime(r.Day),
+		Day:              timestamptzToTime(r.Day),
 		PromptTokens:     int(r.PromptTokens),
 		CompletionTokens: int(r.CompletionTokens),
 		ThinkingTokens:   int(r.ThinkingTokens),
@@ -1142,7 +1133,7 @@ func (r *pgRepo) SummarizeUserDaily(ctx context.Context, userID uuid.UUID, from,
 	out := make([]DayUsageRow, 0, len(rows))
 	for _, row := range rows {
 		out = append(out, DayUsageRow{
-			Day:              intervalToTime(row.Day),
+			Day:              timestamptzToTime(row.Day),
 			PromptTokens:     int(row.PromptTokens),
 			CompletionTokens: int(row.CompletionTokens),
 			ThinkingTokens:   int(row.ThinkingTokens),
diff --git a/internal/chat/sqlc/usage.sql.go b/internal/chat/sqlc/usage.sql.go
index f4aaa09..f42661d 100644
--- a/internal/chat/sqlc/usage.sql.go
+++ b/internal/chat/sqlc/usage.sql.go
@@ -46,7 +46,7 @@ func (q *Queries) InsertUsage(ctx context.Context, arg InsertUsageParams) error
 }
 
 const summarizeUsageByDay = `-- name: SummarizeUsageByDay :many
-SELECT date_trunc('day', created_at) AS day,
+SELECT date_trunc('day', created_at)::timestamptz AS day,
        SUM(prompt_tokens)::int     AS prompt_tokens,
        SUM(completion_tokens)::int AS completion_tokens,
        SUM(thinking_tokens)::int   AS thinking_tokens,
@@ -63,11 +63,11 @@ type SummarizeUsageByDayParams struct {
 }
 
 type SummarizeUsageByDayRow struct {
-	Day              pgtype.Interval `json:"day"`
-	PromptTokens     int32           `json:"prompt_tokens"`
-	CompletionTokens int32           `json:"completion_tokens"`
-	ThinkingTokens   int32           `json:"thinking_tokens"`
-	CostUsd          pgtype.Numeric  `json:"cost_usd"`
+	Day              pgtype.Timestamptz `json:"day"`
+	PromptTokens     int32              `json:"prompt_tokens"`
+	CompletionTokens int32              `json:"completion_tokens"`
+	ThinkingTokens   int32              `json:"thinking_tokens"`
+	CostUsd          pgtype.Numeric     `json:"cost_usd"`
 }
 
 func (q *Queries) SummarizeUsageByDay(ctx context.Context, arg SummarizeUsageByDayParams) ([]SummarizeUsageByDayRow, error) {
@@ -199,7 +199,7 @@ func (q *Queries) SummarizeUsageByUser(ctx context.Context, arg SummarizeUsageBy
 }
 
 const summarizeUserDaily = `-- name: SummarizeUserDaily :many
-SELECT date_trunc('day', created_at) AS day,
+SELECT date_trunc('day', created_at)::timestamptz AS day,
        SUM(prompt_tokens)::int     AS prompt_tokens,
        SUM(completion_tokens)::int AS completion_tokens,
        SUM(thinking_tokens)::int   AS thinking_tokens,
@@ -217,11 +217,11 @@ type SummarizeUserDailyParams struct {
 }
 
 type SummarizeUserDailyRow struct {
-	Day              pgtype.Interval `json:"day"`
-	PromptTokens     int32           `json:"prompt_tokens"`
-	CompletionTokens int32           `json:"completion_tokens"`
-	ThinkingTokens   int32           `json:"thinking_tokens"`
-	CostUsd          pgtype.Numeric  `json:"cost_usd"`
+	Day              pgtype.Timestamptz `json:"day"`
+	PromptTokens     int32              `json:"prompt_tokens"`
+	CompletionTokens int32              `json:"completion_tokens"`
+	ThinkingTokens   int32              `json:"thinking_tokens"`
+	CostUsd          pgtype.Numeric     `json:"cost_usd"`
 }
 
 func (q *Queries) SummarizeUserDaily(ctx context.Context, arg SummarizeUserDailyParams) ([]SummarizeUserDailyRow, error) {