diff --git a/devel/210_15.md b/devel/210_15.md
index 5ada9e35..da986b66 100644
--- a/devel/210_15.md
+++ b/devel/210_15.md
@@ -1,5 +1,7 @@
 # [210_15] srfi-19 time-utc date相互转化实现
 
+## 修改 srfi-19 time-utc date相互转化实现，添加测试
+
 ## 添加 srfi-19 time-utc date相互转化实现
 
 ## 如何测试
diff --git a/goldfish/srfi/srfi-19.scm b/goldfish/srfi/srfi-19.scm
index 7fde8c4b..95b75721 100644
--- a/goldfish/srfi/srfi-19.scm
+++ b/goldfish/srfi/srfi-19.scm
@@ -418,16 +418,20 @@
     ;; Time/Date/Julian Day/Modified Julian Day Converters
     ;; ====================
 
-    (define (priv:days-before-year year)
-      (+ (* 365 year)
-         (floor-quotient year 4)
-         (- (floor-quotient year 100))
-         (floor-quotient year 400)))
-
     (define (priv:days-since-epoch year month day)
-      (+ (- (priv:days-before-year year)
-            (priv:days-before-year 1970))
-         (- (priv:year-day day month year) 1)))
+      ;; Howard Hinnant's days_from_civil algorithm, inverse of civil-from-days
+      (let* ((y (- year (if (<= month 2) 1 0)))
+             (era (if (>= y 0)
+                    (floor-quotient y 400)
+                    (floor-quotient (- y 399) 400)))
+             (yoe (- y (* era 400)))
+             (m (+ month (if (> month 2) -3 9))) ; March=0, ..., February=11
+             (doy (+ (floor-quotient (+ (* 153 m) 2) 5) (- day 1)))
+             (doe (+ (* yoe 365)
+                     (floor-quotient yoe 4)
+                     (- (floor-quotient yoe 100))
+                     doy)))
+        (- (+ (* era 146097) doe) 719468)))
 
     (define (priv:civil-from-days days)
       ;; Howard Hinnant's algorithm, adapted for proleptic Gregorian calendar
@@ -450,6 +454,8 @@
              (y (if (<= m 2) (+ y 1) y)))
         (values y m d)))
 
+    ;; TODO: spec says default tz-offset should be local time zone.
+    ;; We don't have a local tz interface yet, so default is 0 (UTC).
     (define* (time-utc->date time-utc (tz-offset 0))
       (unless (and (time? time-utc) (eq? (time-type time-utc) TIME-UTC))
         (error 'wrong-type-arg "time-utc->date: time-utc must be a TIME-UTC object" time-utc))
diff --git a/tests/goldfish/liii/time-test.scm b/tests/goldfish/liii/time-test.scm
index d021e9a3..1a2fd2b2 100644
--- a/tests/goldfish/liii/time-test.scm
+++ b/tests/goldfish/liii/time-test.scm
@@ -849,6 +849,37 @@ time-utc->date
 
 date->time-utc
 将日期对象转换为 TIME-UTC 时间对象。
+
+语法
+----
+(time-utc->date time-utc [tz-offset])
+(date->time-utc date)
+
+参数
+----
+time-utc : time?
+必须是 TIME-UTC 类型的时间对象。
+
+tz-offset : integer? (可选)
+时区偏移（秒），默认 0 表示 UTC。
+
+date : date?
+日期对象。
+
+返回值
+-----
+time-utc->date : date?
+date->time-utc : time?
+
+说明
+----
+1. time-utc->date 将 UTC 时间按 tz-offset 转换成本地日期。
+2. date->time-utc 将本地日期按 date 的 zone-offset 转回 UTC 时间。
+
+错误处理
+--------
+wrong-type-arg
+当参数类型不正确，或 time-utc 不是 TIME-UTC 时抛出错误。
 |#
 
 ;; time-utc->date basic (UTC)
@@ -933,6 +964,78 @@ date->time-utc
   (check (date-nanosecond d2) => (date-nanosecond d1))
   (check (date-zone-offset d2) => (date-zone-offset d1)))
 
+;; 2000-02-29 leap day round-trip (UTC)
+(let* ((d1 (make-date 0 0 0 0 29 2 2000 0))
+       (t (date->time-utc d1))
+       (d2 (time-utc->date t 0)))
+  (check (date-year d2) => 2000)
+  (check (date-month d2) => 2)
+  (check (date-day d2) => 29))
+
+;; additional leap year boundary cases (UTC)
+(let* ((d1 (make-date 0 0 0 0 28 2 1900 0)) ; 1900 is not a leap year
+       (t (date->time-utc d1))
+       (d2 (time-utc->date t 0)))
+  (check (date-year d2) => 1900)
+  (check (date-month d2) => 2)
+  (check (date-day d2) => 28))
+
+(let* ((d1 (make-date 0 0 0 0 1 3 1900 0))
+       (t (date->time-utc d1))
+       (d2 (time-utc->date t 0)))
+  (check (date-year d2) => 1900)
+  (check (date-month d2) => 3)
+  (check (date-day d2) => 1))
+
+(let* ((d1 (make-date 0 0 0 0 29 2 2004 0)) ; regular leap year
+       (t (date->time-utc d1))
+       (d2 (time-utc->date t 0)))
+  (check (date-year d2) => 2004)
+  (check (date-month d2) => 2)
+  (check (date-day d2) => 29))
+
+(let* ((d1 (make-date 0 0 0 0 28 2 2100 0)) ; 2100 is not a leap year
+       (t (date->time-utc d1))
+       (d2 (time-utc->date t 0)))
+  (check (date-year d2) => 2100)
+  (check (date-month d2) => 2)
+  (check (date-day d2) => 28))
+
+(let* ((d1 (make-date 0 0 0 0 1 3 2100 0))
+       (t (date->time-utc d1))
+       (d2 (time-utc->date t 0)))
+  (check (date-year d2) => 2100)
+  (check (date-month d2) => 3)
+  (check (date-day d2) => 1))
+
+(let* ((d1 (make-date 0 0 0 0 29 2 2400 0)) ; 2400 is a leap year
+       (t (date->time-utc d1))
+       (d2 (time-utc->date t 0)))
+  (check (date-year d2) => 2400)
+  (check (date-month d2) => 2)
+  (check (date-day d2) => 29))
+
+;; time-utc -> date -> time-utc round-trip cases
+(let* ((t1 (make-time TIME-UTC 0 0))
+       (d (time-utc->date t1))
+       (t2 (date->time-utc d)))
+  (check-true (time=? t1 t2)))
+
+(let* ((t1 (make-time TIME-UTC 123456789 98765))
+       (d (time-utc->date t1))
+       (t2 (date->time-utc d)))
+  (check-true (time=? t1 t2)))
+
+(let* ((t1 (make-time TIME-UTC 500000000 -12345))
+       (d (time-utc->date t1))
+       (t2 (date->time-utc d)))
+  (check-true (time=? t1 t2)))
+
+(let* ((t1 (make-time TIME-UTC 0 1704067200))
+       (d (time-utc->date t1))
+       (t2 (date->time-utc d)))
+  (check-true (time=? t1 t2)))
+
 ;; converter error conditions
 (check-catch 'wrong-type-arg
   (time-utc->date (make-time TIME-TAI 0 0) 0))