diff --git a/src/trustme/__init__.py b/src/trustme/__init__.py
index bd837f8..5fb24fb 100644
--- a/src/trustme/__init__.py
+++ b/src/trustme/__init__.py
@@ -35,6 +35,8 @@
 # ~3001-01-19:
 #   https://github.com/pyca/cryptography/issues/3194
 DEFAULT_EXPIRY = datetime.datetime(3000, 1, 1)
+DEFAULT_NOT_BEFORE = datetime.datetime(2000, 1, 1)
+
 
 def _name(name: str, organization_name: Optional[str] = None, common_name: Optional[str] = None) -> x509.Name:
     name_pieces = [
@@ -64,14 +66,16 @@ def _cert_builder_common(
     issuer: x509.Name,
     public_key: CERTIFICATE_PUBLIC_KEY_TYPES,
     not_after: Optional[datetime.datetime] = None,
+    not_before: Optional[datetime.datetime] = None,
 ) -> x509.CertificateBuilder:
     not_after = not_after if not_after else DEFAULT_EXPIRY
+    not_before = not_before if not_before else DEFAULT_NOT_BEFORE
     return (
         x509.CertificateBuilder()
         .subject_name(subject)
         .issuer_name(issuer)
         .public_key(public_key)
-        .not_valid_before(datetime.datetime(2000, 1, 1))
+        .not_valid_before(not_before)
         .not_valid_after(not_after)
         .serial_number(x509.random_serial_number())
         .add_extension(
@@ -311,6 +315,7 @@ def issue_cert(
         common_name: Optional[str] = None,
         organization_name: Optional[str] = None,
         organization_unit_name: Optional[str] = None,
+        not_before: Optional[datetime.datetime] = None,
         not_after: Optional[datetime.datetime] = None,
         key_type: KeyType = KeyType.ECDSA,
     ) -> "LeafCert":
@@ -351,6 +356,9 @@ def issue_cert(
             attribute on the certificate. By default, a random one will be
             generated.
 
+          not_before: Set the validity start date (notBefore) of the certificate.
+            This argument type is `datetime.datetime`.
+
           not_after: Set the expiry date (notAfter) of the certificate. This
             argument type is `datetime.datetime`.
 
@@ -380,6 +388,7 @@ def issue_cert(
                 ),
                 self._certificate.subject,
                 key.public_key(),
+                not_before=not_before,
                 not_after=not_after,
             )
             .add_extension(
diff --git a/tests/test_trustme.py b/tests/test_trustme.py
index 3360336..1d901ad 100644
--- a/tests/test_trustme.py
+++ b/tests/test_trustme.py
@@ -171,6 +171,23 @@ def test_issue_cert_custom_not_after() -> None:
         assert getattr(cert.not_valid_after, t) == getattr(expires, t)
 
 
+def test_issue_cert_custom_not_before() -> None:
+    not_before = datetime.datetime(2027, 7, 5, 17, 15, 30)
+    ca = CA()
+
+    leaf_cert = ca.issue_cert(
+        "example.org",
+        organization_name="python-trio",
+        organization_unit_name="trustme",
+        not_before=not_before,
+    )
+
+    cert = x509.load_pem_x509_certificate(leaf_cert.cert_chain_pems[0].bytes())
+
+    for t in ["year", "month", "day", "hour", "minute", "second"]:
+        assert getattr(cert.not_valid_before, t) == getattr(not_before, t)
+
+
 def test_intermediate() -> None:
     ca = CA()
     ca_cert = x509.load_pem_x509_certificate(ca.cert_pem.bytes())