diff --git a/src/Surfnet/Stepup/DateTime/DateTime.php b/src/Surfnet/Stepup/DateTime/DateTime.php
index 5ef0b245a..4c1e4f85b 100644
--- a/src/Surfnet/Stepup/DateTime/DateTime.php
+++ b/src/Surfnet/Stepup/DateTime/DateTime.php
@@ -78,13 +78,13 @@ public function __construct(CoreDateTime $dateTime = null)
     }
 
     /**
-     * @param string $intervalSpec
+     * @param DateInterval $intervalSpec
      * @return DateTime
      */
-    public function add($intervalSpec)
+    public function add(DateInterval $intervalSpec)
     {
         $dateTime = clone $this->dateTime;
-        $dateTime->add(new DateInterval($intervalSpec));
+        $dateTime->add($intervalSpec);
 
         return new self($dateTime);
     }
diff --git a/src/Surfnet/Stepup/Identity/Entity/UnverifiedSecondFactor.php b/src/Surfnet/Stepup/Identity/Entity/UnverifiedSecondFactor.php
index 8bc9d1a7f..f6103064f 100644
--- a/src/Surfnet/Stepup/Identity/Entity/UnverifiedSecondFactor.php
+++ b/src/Surfnet/Stepup/Identity/Entity/UnverifiedSecondFactor.php
@@ -19,6 +19,7 @@
 namespace Surfnet\Stepup\Identity\Entity;
 
 use Broadway\EventSourcing\EventSourcedEntity;
+use DateInterval;
 use Surfnet\Stepup\DateTime\DateTime;
 use Surfnet\Stepup\Exception\DomainException;
 use Surfnet\Stepup\Exception\InvalidArgumentException;
@@ -123,7 +124,7 @@ public function getId()
     public function wouldVerifyEmail($verificationNonce)
     {
         return $this->verificationNonce === $verificationNonce
-            && !DateTime::now()->comesAfter($this->verificationRequestedAt->add('P1D'));
+            && !DateTime::now()->comesAfter($this->verificationRequestedAt->add(new DateInterval('P1D')));
     }
 
     /**
@@ -140,7 +141,7 @@ public function verifyEmail($verificationNonce)
             );
         }
 
-        if (DateTime::now()->comesAfter($this->verificationRequestedAt->add('P1D'))) {
+        if (DateTime::now()->comesAfter($this->verificationRequestedAt->add(new DateInterval('P1D')))) {
             throw new DomainException(
                 sprintf(
                     "Cannot verify possession of e-mail for second factor '%s': " .
diff --git a/src/Surfnet/Stepup/Identity/Entity/VerifiedSecondFactor.php b/src/Surfnet/Stepup/Identity/Entity/VerifiedSecondFactor.php
index f4d51fc5e..49f42cf26 100644
--- a/src/Surfnet/Stepup/Identity/Entity/VerifiedSecondFactor.php
+++ b/src/Surfnet/Stepup/Identity/Entity/VerifiedSecondFactor.php
@@ -116,7 +116,7 @@ public function wouldBeVettedBy($registrationCode, $secondFactorIdentifier, $doc
         return strcasecmp($registrationCode, $this->registrationCode) === 0
             && $secondFactorIdentifier === $this->secondFactorIdentifier
             && $identityVerified === true
-            && !DateTime::now()->comesAfter($this->registrationRequestedAt->add('P14D'));
+            && !DateTime::now()->comesAfter($this->registrationRequestedAt->add(new \DateInterval('P14D')));
     }
 
     /**
@@ -139,7 +139,7 @@ public function vet($registrationCode, $secondFactorIdentifier, $documentNumber,
             throw new DomainException("Cannot vet this second factor: real identity wasn't verified by an RA.");
         }
 
-        if (DateTime::now()->comesAfter($this->registrationRequestedAt->add('P14D'))) {
+        if (DateTime::now()->comesAfter($this->registrationRequestedAt->add(new \DateInterval('P14D')))) {
             throw new DomainException('Cannot vet this second factor: registration window has closed.');
         }
 
diff --git a/src/Surfnet/StepupMiddleware/CommandHandlingBundle/Tests/DateTimeHelper.php b/src/Surfnet/StepupMiddleware/CommandHandlingBundle/Tests/DateTimeHelper.php
index 61cdb747e..3089d4085 100644
--- a/src/Surfnet/StepupMiddleware/CommandHandlingBundle/Tests/DateTimeHelper.php
+++ b/src/Surfnet/StepupMiddleware/CommandHandlingBundle/Tests/DateTimeHelper.php
@@ -28,7 +28,7 @@ class DateTimeHelper
      *
      * @param DateTime|null $now
      */
-    public static function stubNow(DateTime $now = null)
+    public static function setCurrentTime(DateTime $now = null)
     {
         $nowProperty = new ReflectionProperty('Surfnet\Stepup\DateTime\DateTime', 'now');
         $nowProperty->setAccessible(true);
diff --git a/src/Surfnet/StepupMiddleware/CommandHandlingBundle/Tests/DateTimeHelperTest.php b/src/Surfnet/StepupMiddleware/CommandHandlingBundle/Tests/DateTimeHelperTest.php
index 0a7d96a7a..ef266ed9f 100644
--- a/src/Surfnet/StepupMiddleware/CommandHandlingBundle/Tests/DateTimeHelperTest.php
+++ b/src/Surfnet/StepupMiddleware/CommandHandlingBundle/Tests/DateTimeHelperTest.php
@@ -32,7 +32,7 @@ class DateTimeHelperTest extends \PHPUnit_Framework_TestCase
      */
     public function it_mocks_now()
     {
-        DateTimeHelper::stubNow(new DateTime(new CoreDateTime('@12345')));
+        DateTimeHelper::setCurrentTime(new DateTime(new CoreDateTime('@12345')));
 
         $this->assertEquals(new DateTime(new CoreDateTime('@12345')), DateTime::now());
     }
@@ -43,10 +43,10 @@ public function it_mocks_now()
      */
     public function it_can_be_disabled_in_the_same_process()
     {
-        DateTimeHelper::stubNow(new DateTime(new CoreDateTime('@12345')));
+        DateTimeHelper::setCurrentTime(new DateTime(new CoreDateTime('@12345')));
         $this->assertEquals(new DateTime(new CoreDateTime('@12345')), DateTime::now());
 
-        DateTimeHelper::stubNow(null);
+        DateTimeHelper::setCurrentTime(null);
         $this->assertTrue((new DateTime())->comesAfterOrIsEqual(DateTime::now()));
     }
 
diff --git a/src/Surfnet/StepupMiddleware/CommandHandlingBundle/Tests/Identity/CommandHandler/IdentityCommandHandlerTest.php b/src/Surfnet/StepupMiddleware/CommandHandlingBundle/Tests/Identity/CommandHandler/IdentityCommandHandlerTest.php
index 5eb425d8f..aa06df685 100644
--- a/src/Surfnet/StepupMiddleware/CommandHandlingBundle/Tests/Identity/CommandHandler/IdentityCommandHandlerTest.php
+++ b/src/Surfnet/StepupMiddleware/CommandHandlingBundle/Tests/Identity/CommandHandler/IdentityCommandHandlerTest.php
@@ -47,6 +47,9 @@
 use Surfnet\StepupMiddleware\CommandHandlingBundle\Identity\CommandHandler\IdentityCommandHandler;
 use Surfnet\StepupMiddleware\CommandHandlingBundle\Tests\DateTimeHelper;
 
+/**
+ * @runTestsInSeparateProcesses
+ */
 class IdentityCommandHandlerTest extends CommandHandlerTest
 {
     /** @var MockInterface */
@@ -79,7 +82,7 @@ protected function createCommandHandler(EventStoreInterface $eventStore, EventBu
      */
     public function a_yubikey_possession_can_be_proven()
     {
-        DateTimeHelper::stubNow(new DateTime(new CoreDateTime('@12345')));
+        DateTimeHelper::setCurrentTime(new DateTime(new CoreDateTime('@12345')));
 
         m::mock('alias:Surfnet\Stepup\Token\TokenGenerator')
             ->shouldReceive('generateHumanReadableToken')->once()->andReturn('code')
@@ -162,7 +165,7 @@ public function yubikey_possession_cannot_be_proven_twice()
      */
     public function a_phone_possession_can_be_proven()
     {
-        DateTimeHelper::stubNow(new DateTime(new CoreDateTime('@12345')));
+        DateTimeHelper::setCurrentTime(new DateTime(new CoreDateTime('@12345')));
 
         m::mock('alias:Surfnet\Stepup\Token\TokenGenerator')
             ->shouldReceive('generateHumanReadableToken')->once()->andReturn('code')
@@ -285,7 +288,7 @@ public function cannot_prove_possession_of_arbitrary_second_factor_type_twice()
      */
     public function an_unverified_second_factors_email_can_be_verified()
     {
-        DateTimeHelper::stubNow(new DateTime(new CoreDateTime('@12345')));
+        DateTimeHelper::setCurrentTime(new DateTime(new CoreDateTime('@12345')));
 
         m::mock('alias:Surfnet\Stepup\Token\TokenGenerator')
             ->shouldReceive('generateHumanReadableToken')->once()->andReturn('regcode');