diff --git a/src/ChronosDate.php b/src/ChronosDate.php
index b51d911..ab63951 100644
--- a/src/ChronosDate.php
+++ b/src/ChronosDate.php
@@ -300,7 +300,7 @@ public static function diffFormatter(?DifferenceFormatterInterface $formatter =
      */
     public function add(DateInterval $interval): static
     {
-        if ($interval->f > 0 || $interval->s > 0 || $interval->i > 0 || $interval->h > 0) {
+        if ($interval->f != 0 || $interval->s !== 0 || $interval->i !== 0 || $interval->h !== 0) {
             throw new InvalidArgumentException('Cannot add intervals with time components');
         }
         $new = clone $this;
@@ -319,7 +319,7 @@ public function add(DateInterval $interval): static
      */
     public function sub(DateInterval $interval): static
     {
-        if ($interval->f > 0 || $interval->s > 0 || $interval->i > 0 || $interval->h > 0) {
+        if ($interval->f != 0 || $interval->s !== 0 || $interval->i !== 0 || $interval->h !== 0) {
             throw new InvalidArgumentException('Cannot subtract intervals with time components');
         }
         $new = clone $this;
@@ -338,7 +338,21 @@ public function sub(DateInterval $interval): static
      */
     public function modify(string $modifier): static
     {
-        if (preg_match('/hour|minute|second/', $modifier)) {
+        $parsed = date_parse($modifier);
+        $relative = $parsed['relative'] ?? [];
+        $timestampSeconds = 0;
+        if (preg_match('/(?:^|\s)@(-?\d+)(?:\.\d+)?(?:\s|$)/', $modifier, $matches)) {
+            $timestampSeconds = (int)$matches[1];
+        }
+        if (
+            ($relative['hour'] ?? 0) !== 0 ||
+            ($relative['minute'] ?? 0) !== 0 ||
+            ($relative['second'] ?? 0) !== $timestampSeconds ||
+            preg_match(
+                '/(?<![[:alpha:]])(?:milliseconds?|microseconds?|msecs?|usecs?|µsecs?|ms|µs)(?![[:alpha:]])/iu',
+                $modifier,
+            )
+        ) {
             throw new InvalidArgumentException('Cannot modify date objects by time values');
         }
 
diff --git a/tests/TestCase/Date/AddTest.php b/tests/TestCase/Date/AddTest.php
index ba831b5..dba25f7 100644
--- a/tests/TestCase/Date/AddTest.php
+++ b/tests/TestCase/Date/AddTest.php
@@ -37,6 +37,14 @@ public function testAddRaiseErrorOnTime(): void
         $date->add($interval);
     }
 
+    public function testAddRaiseErrorOnNegativeTime(): void
+    {
+        $interval = DateInterval::createFromDateString('-1 hour');
+        $date = ChronosDate::create(2001, 1, 1);
+        $this->expectException(InvalidArgumentException::class);
+        $date->add($interval);
+    }
+
     public function testSubFullDay(): void
     {
         $interval = DateInterval::createFromDateString('1 day');
@@ -53,6 +61,14 @@ public function testSubIgnoreTime(): void
         $date->sub($interval);
     }
 
+    public function testSubRaiseErrorOnNegativeTime(): void
+    {
+        $interval = DateInterval::createFromDateString('-1 hour');
+        $date = ChronosDate::create(2001, 1, 1);
+        $this->expectException(InvalidArgumentException::class);
+        $date->sub($interval);
+    }
+
     public function testAddDay(): void
     {
         $this->assertSame(1, ChronosDate::create(1975, 5, 31)->addDays(1)->day);
@@ -90,4 +106,76 @@ public function testModifyTimeComponentError(): void
         $this->expectException(InvalidArgumentException::class);
         $date->modify('10 seconds');
     }
+
+    public function testModifyOrdinalWeekday(): void
+    {
+        $date = ChronosDate::create(2026, 9, 1);
+        $new = $date->modify('second monday of this month');
+        $this->assertSame('2026-09-14', $new->toDateString());
+    }
+
+    public function testModifyUppercaseTimeComponentError(): void
+    {
+        $date = ChronosDate::create(2001, 1, 1);
+        $this->expectException(InvalidArgumentException::class);
+        $date->modify('10 SECONDS');
+    }
+
+    public function testModifyClockTimeIgnored(): void
+    {
+        $date = ChronosDate::create(2001, 1, 1);
+        $new = $date->modify('12:00');
+        $this->assertSame('2001-01-01', $new->toDateString());
+    }
+
+    public function testModifyTimeComponentWrappingDayError(): void
+    {
+        $date = ChronosDate::create(2001, 1, 1);
+        $this->expectException(InvalidArgumentException::class);
+        $date->modify('+24 hours');
+    }
+
+    public function testModifyFractionalTimeComponentError(): void
+    {
+        $date = ChronosDate::create(2001, 1, 1);
+        $this->expectException(InvalidArgumentException::class);
+        $date->modify('1 millisecond');
+    }
+
+    public function testModifyAbbreviatedFractionalTimeComponentError(): void
+    {
+        $date = ChronosDate::create(2001, 1, 1);
+        $this->expectException(InvalidArgumentException::class);
+        $date->modify('-1 msec');
+    }
+
+    public function testModifyTimestamp(): void
+    {
+        $date = ChronosDate::create(2001, 1, 1);
+        $new = $date->modify('@86400');
+        $this->assertSame('1970-01-02', $new->toDateString());
+    }
+
+    public function testModifyTimestampWithTimezone(): void
+    {
+        $date = ChronosDate::create(2001, 1, 1);
+        $new = $date->modify('@1 UTC');
+        $this->assertSame('1970-01-01', $new->toDateString());
+    }
+
+    public function testModifyTimestampWithDateComponent(): void
+    {
+        $date = ChronosDate::create(2001, 1, 1);
+        $new = $date->modify('@86400 +1 day');
+        $this->assertSame('1970-01-03', $new->toDateString());
+    }
+
+    public function testModifyAcrossSkippedMidnight(): void
+    {
+        $this->withTimezone('America/Havana', function (): void {
+            $date = new ChronosDate('2023-03-11');
+            $new = $date->modify('+1 day');
+            $this->assertSame('2023-03-12', $new->toDateString());
+        });
+    }
 }