diff --git a/src/date.rs b/src/date.rs
index e28b037d9..efbdfe1e0 100644
--- a/src/date.rs
+++ b/src/date.rs
@@ -198,6 +198,9 @@ impl Date {
     pub(crate) const fn from_julian_day_unchecked(julian_day: i32) -> Self {
         #![allow(trivial_numeric_casts)] // cast depends on type alias
 
+        debug_assert!(julian_day >= Self::MIN.to_julian_day());
+        debug_assert!(julian_day <= Self::MAX.to_julian_day());
+
         /// A type that is either `i32` or `i64`. This subtle difference allows for optimization
         /// based on the valid values.
         #[cfg(feature = "large-dates")]
@@ -451,7 +454,10 @@ impl Date {
             -3 | 4 => Weekday::Friday,
             -2 | 5 => Weekday::Saturday,
             -1 | 6 => Weekday::Sunday,
-            _ => Weekday::Monday,
+            val => {
+                debug_assert!(val == 0);
+                Weekday::Monday
+            }
         }
     }
 
@@ -664,6 +670,7 @@ impl Date {
         } else if duration.is_negative() {
             Self::MIN
         } else {
+            debug_assert!(duration.is_positive());
             Self::MAX
         }
     }
@@ -701,6 +708,7 @@ impl Date {
         } else if duration.is_negative() {
             Self::MAX
         } else {
+            debug_assert!(duration.is_positive());
             Self::MIN
         }
     }
diff --git a/src/duration.rs b/src/duration.rs
index a765dffbe..91aaeb938 100644
--- a/src/duration.rs
+++ b/src/duration.rs
@@ -201,6 +201,16 @@ impl Duration {
     // region: constructors
     /// Create a new `Duration` without checking the validity of the components.
     pub(crate) const fn new_unchecked(seconds: i64, nanoseconds: i32) -> Self {
+        if seconds < 0 {
+            debug_assert!(nanoseconds <= 0);
+            debug_assert!(nanoseconds > -1_000_000_000);
+        } else if seconds > 0 {
+            debug_assert!(nanoseconds >= 0);
+            debug_assert!(nanoseconds < 1_000_000_000);
+        } else {
+            debug_assert!(nanoseconds.unsigned_abs() < 1_000_000_000);
+        }
+
         Self {
             seconds,
             nanoseconds,
diff --git a/src/instant.rs b/src/instant.rs
index f2044a39b..7f95a694e 100644
--- a/src/instant.rs
+++ b/src/instant.rs
@@ -166,6 +166,7 @@ impl Add<Duration> for Instant {
         } else if duration.is_negative() {
             Self(self.0 - duration.unsigned_abs())
         } else {
+            debug_assert!(duration.is_zero());
             self
         }
     }
@@ -199,6 +200,7 @@ impl Sub<Duration> for Instant {
         } else if duration.is_negative() {
             Self(self.0 + duration.unsigned_abs())
         } else {
+            debug_assert!(duration.is_zero());
             self
         }
     }
diff --git a/src/offset_date_time.rs b/src/offset_date_time.rs
index 0342eb548..59860b58c 100644
--- a/src/offset_date_time.rs
+++ b/src/offset_date_time.rs
@@ -760,6 +760,7 @@ impl OffsetDateTime {
                 .assume_utc()
                 .replace_offset(self.offset)
         } else {
+            debug_assert!(duration.is_positive());
             PrimitiveDateTime::MAX
                 .assume_utc()
                 .replace_offset(self.offset)
@@ -794,6 +795,7 @@ impl OffsetDateTime {
                 .assume_utc()
                 .replace_offset(self.offset)
         } else {
+            debug_assert!(duration.is_positive());
             PrimitiveDateTime::MIN
                 .assume_utc()
                 .replace_offset(self.offset)
diff --git a/src/quickcheck.rs b/src/quickcheck.rs
index 17e34df2b..22241914d 100644
--- a/src/quickcheck.rs
+++ b/src/quickcheck.rs
@@ -176,7 +176,10 @@ impl Arbitrary for Weekday {
             3 => Thursday,
             4 => Friday,
             5 => Saturday,
-            _ => Sunday,
+            val => {
+                debug_assert!(val == 6);
+                Sunday
+            }
         }
     }
 
@@ -203,7 +206,10 @@ impl Arbitrary for Month {
             9 => September,
             10 => October,
             11 => November,
-            _ => December,
+            val => {
+                debug_assert!(val == 12);
+                December
+            }
         }
     }
 
diff --git a/src/rand.rs b/src/rand.rs
index ff26d8af0..8afefe507 100644
--- a/src/rand.rs
+++ b/src/rand.rs
@@ -67,7 +67,10 @@ impl Distribution<Weekday> for Standard {
             3 => Thursday,
             4 => Friday,
             5 => Saturday,
-            _ => Sunday,
+            val => {
+                debug_assert!(val == 6);
+                Sunday
+            }
         }
     }
 }
@@ -87,7 +90,10 @@ impl Distribution<Month> for Standard {
             9 => September,
             10 => October,
             11 => November,
-            _ => December,
+            val => {
+                debug_assert!(val == 12);
+                December
+            }
         }
     }
 }
diff --git a/src/time.rs b/src/time.rs
index e635dadce..7ef6fa87c 100644
--- a/src/time.rs
+++ b/src/time.rs
@@ -81,6 +81,11 @@ impl Time {
         second: u8,
         nanosecond: u32,
     ) -> Self {
+        debug_assert!(hour < 24);
+        debug_assert!(minute < 60);
+        debug_assert!(second < 60);
+        debug_assert!(nanosecond < 1_000_000_000);
+
         Self {
             hour,
             minute,
diff --git a/src/utc_offset.rs b/src/utc_offset.rs
index 65ae5fa41..5f3b72f9b 100644
--- a/src/utc_offset.rs
+++ b/src/utc_offset.rs
@@ -45,6 +45,22 @@ impl UtcOffset {
     /// sign.
     #[doc(hidden)]
     pub const fn __from_hms_unchecked(hours: i8, minutes: i8, seconds: i8) -> Self {
+        if hours < 0 {
+            debug_assert!(minutes <= 0);
+            debug_assert!(seconds <= 0);
+        } else if hours > 0 {
+            debug_assert!(minutes >= 0);
+            debug_assert!(seconds >= 0);
+        }
+        if minutes < 0 {
+            debug_assert!(seconds <= 0);
+        } else if minutes > 0 {
+            debug_assert!(seconds >= 0);
+        }
+        debug_assert!(hours.unsigned_abs() < 24);
+        debug_assert!(minutes.unsigned_abs() < 60);
+        debug_assert!(seconds.unsigned_abs() < 60);
+
         Self {
             hours,
             minutes,