diff --git a/src/edutap/wallet_google/api.py b/src/edutap/wallet_google/api.py
index b939afb..2ca7739 100644
--- a/src/edutap/wallet_google/api.py
+++ b/src/edutap/wallet_google/api.py
@@ -339,6 +339,21 @@ def listing(
         break
 
 
+def _convert_str_or_datetime_to_str(value: str | datetime.datetime) -> str:
+    """convert and check the value to be a valid string for the JWT claim timestamps"""
+    if isinstance(value, datetime.datetime):
+        return str(int(value.timestamp()))
+    if value == "":
+        return value
+    if not value.isdecimal():
+        raise ValueError("string must be a decimal")
+    if int(value) < 0:
+        raise ValueError("string must be an int >= 0 number")
+    if int(value) > 2**32:
+        raise ValueError("string must be an int < 2**32 number")
+    return value
+
+
 def save_link(
     models: list[ClassModel | ObjectModel | Reference],
     *,
@@ -363,6 +378,8 @@ def save_link(
                         The Google Wallet API button will not render when the origins field is not defined.
                         You could potentially get an "Load denied by X-Frame-Options" or "Refused to display"
                         messages in the browser console when the origins field is not defined.
+    :param: iat:        Issued At Time. The time when the JWT was issued.
+    :param: exp:        Expiration Time. The time when the JWT expires.
     :return:            Link with JWT to save the resources to the wallet.
     """
     payload = JWTPayload()
@@ -378,26 +395,12 @@ def save_link(
             setattr(payload, name, [])
         getattr(payload, name).append(model)
 
-    if isinstance(iat, datetime.datetime):
-        iat = str(int(iat.timestamp()))
-    if iat != "":
-        assert iat.isdecimal()
-        assert int(iat) >= 0
-        assert int(iat) < 2**32
-
-    if isinstance(exp, datetime.datetime):
-        exp = str(int(exp.timestamp()))
-    if exp != "":
-        assert exp.isdecimal()
-        assert int(exp) >= 0
-        assert int(exp) < 2**32
-
     claims = JWTClaims(
         iss=session_manager.settings.credentials_info["client_email"],
         origins=origins,
         payload=payload,
-        iat=iat,
-        exp=exp,
+        iat=_convert_str_or_datetime_to_str(iat),
+        exp=_convert_str_or_datetime_to_str(exp),
     )
 
     logger.debug(claims.model_dump_json(indent=2))
diff --git a/tests/test_api_save_link.py b/tests/test_api_save_link.py
index 253cfaa..09cdab9 100644
--- a/tests/test_api_save_link.py
+++ b/tests/test_api_save_link.py
@@ -1,3 +1,6 @@
+import pytest
+
+
 def test_api_save_link(mock_settings):
     from edutap.wallet_google.settings import ROOT_DIR
 
@@ -19,7 +22,48 @@ def test_api_save_link(mock_settings):
             ),
         ]
     )
-    assert (
-        link
-        == "https://pay.google.com/gp/v/save/eyJ0eXAiOiAiSldUIiwgImFsZyI6ICJSUzI1NiIsICJraWQiOiAiMTIzNDU2Nzg5MGFiY2RlZjEyMzQ1Njc4OTBhYmNkZWYxMjM0NTY3OCJ9.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.LEPJBlt7ic9cPWKUvpoxUWe5yvdK0_kqPlBFkHmqFBfO5eeYN-owTHCElCGhnHeE730D4U3XjQWeZXfcaEAQcdBKB8udoT2Tja7Rw_M8M18kpBrSdGDRKT_uXG_-RkG3uVB30Lu5otlJiX2VOJWg9H6NR7wD_pfUt67cLjiBeMILuIVi-h0CDUV0dObEjnOHrRhj6KeKdfqq6izwwmw4iSQxsaQrDxWZtwCZ__pV5UK54Od6-lNrsBQwz241SDYv9kJTXrImrjRZXdoht6xgwqxg-GcuqUJgcczG-TLyN_9aI4FtA2cz8PCXyKPXnd-_HTe9nohi05dfMDeVWsmP6g"
-    )
+    expected = "https://pay.google.com/gp/v/save/eyJ0eXAiOiAiSldUIiwgImFsZyI6ICJSUzI1NiIsICJraWQiOiAiMTIzNDU2Nzg5MGFiY2RlZjEyMzQ1Njc4OTBhYmNkZWYxMjM0NTY3OCJ9.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.MaHyvgpwdAl9XB0CS6pO2IYoORPM8XADJZama13cYCVuaPN4VOg3k8TA3jOAyj34uqjG3u6Yr2HGGaKYHh7lfKax2L5CcNEJg02zjTsi7qjGpzMZr62CKhuw2JqzQvsIj8Wug4_zE4ErGASAZdKvZr2bpJWPXmmk9JibOuFpotTwpTd6CmLMhtypp_wyHinRX7FzXTT_V4_SA3ZYEblWF7FBz_m9NsbQjXYQRE0yxCY_bQEDydqK2tzOVd3YGiqLK_zZjBA3H3_rtqY6pdPdyUlADIWTU0dJ0sc1sNl0rjGFCgeMdziNhgppjxQ0iWnVyFb2hZO8QEi83aD5A46X9A"
+
+    assert link == expected
+
+
+def test__convert_str_or_datetime_to_str__timestamp():
+    from edutap.wallet_google.api import _convert_str_or_datetime_to_str
+
+    import datetime
+
+    dt = datetime.datetime(2021, 1, 1, 12, 0, 0)
+    expected = '1609498800'
+
+    assert _convert_str_or_datetime_to_str(dt) == expected
+
+
+def test__convert_str_or_datetime_to_str__str_int_lt_zero():
+    from edutap.wallet_google.api import _convert_str_or_datetime_to_str
+
+    import datetime
+
+
+    with pytest.raises(ValueError) as exc:
+        _convert_str_or_datetime_to_str("-1")
+
+
+def test__convert_str_or_datetime_to_str__str_int_tr_4bytes():
+    from edutap.wallet_google.api import _convert_str_or_datetime_to_str
+
+    import datetime
+
+
+    with pytest.raises(ValueError) as exc:
+        _convert_str_or_datetime_to_str(f"{2**32+1}")
+
+def test__convert_str_or_datetime_to_str__not_decimal():
+    from edutap.wallet_google.api import _convert_str_or_datetime_to_str
+
+    import datetime
+
+
+    with pytest.raises(ValueError) as exc:
+        _convert_str_or_datetime_to_str("x 100")
+
+