diff --git a/core/src/main/kotlin/plus/rua/project/ui/DateCheckerScreen.kt b/core/src/main/kotlin/plus/rua/project/ui/DateCheckerScreen.kt
index e876a68..2dd3811 100644
--- a/core/src/main/kotlin/plus/rua/project/ui/DateCheckerScreen.kt
+++ b/core/src/main/kotlin/plus/rua/project/ui/DateCheckerScreen.kt
@@ -745,6 +745,17 @@ private fun ArrowRightIcon(color: Color, modifier: Modifier = Modifier) {
 
 // region Helpers
 
+/**
+ * 将保质期天数钳制到合法范围 [0, +∞)。
+ *
+ * 天数语义上不能为负(到期日不应早于生产日期)。
+ * 无论来自天数输入框还是日期选择器,写入 [ExpiryRow.days] 前都应经过此函数。
+ *
+ * @param days 原始天数
+ * @return 钳制后的天数,最小为 0
+ */
+fun clampExpiryDays(days: Int): Int = days.coerceAtLeast(0)
+
 private fun LocalDate.toEpochMillis(): Long =
     this.atStartOfDayIn(TimeZone.UTC).toEpochMilliseconds()
 
diff --git a/core/src/test/kotlin/plus/rua/project/ui/DateCheckerScreenLogicTest.kt b/core/src/test/kotlin/plus/rua/project/ui/DateCheckerScreenLogicTest.kt
new file mode 100644
index 0000000..660491c
--- /dev/null
+++ b/core/src/test/kotlin/plus/rua/project/ui/DateCheckerScreenLogicTest.kt
@@ -0,0 +1,29 @@
+package plus.rua.project.ui
+
+import kotlin.test.Test
+import kotlin.test.assertEquals
+
+class DateCheckerScreenLogicTest {
+
+    // ---- clampExpiryDays ----
+
+    @Test
+    fun clampExpiryDays_positiveValue_unchanged() {
+        assertEquals(30, clampExpiryDays(30))
+    }
+
+    @Test
+    fun clampExpiryDays_zero_unchanged() {
+        assertEquals(0, clampExpiryDays(0))
+    }
+
+    @Test
+    fun clampExpiryDays_negativeValue_clampedToZero() {
+        assertEquals(0, clampExpiryDays(-1))
+    }
+
+    @Test
+    fun clampExpiryDays_largeNegative_clampedToZero() {
+        assertEquals(0, clampExpiryDays(-365))
+    }
+}