Enforce new expiration date in future

forms.py

@@ -4,6 +4,7 @@ from wtforms import StringField, PasswordField, IntegerField, SubmitField
 from wtforms.fields.html5 import DateField
 from wtforms.validators import DataRequired, Length, EqualTo, InputRequired, \
     ValidationError, NumberRange, Optional
+from datetime import date
 
 from db import get_registration_code, get_registered_user
 
@@ -30,6 +31,11 @@ def username_availability_validator(form: FlaskForm, field):
         raise ValidationError("Username already registered!")
 
 
+def registration_code_expiration_date_validator(form: FlaskForm, field):
+    if field.data <= date.today():
+        raise ValidationError("Expiration Date must be in the future!")
+
+
 class RegistrationForm(FlaskForm):
     username = StringField(
         'Username',
@@ -56,7 +62,9 @@ class LoginForm(FlaskForm):
 
 
 class RegistrationCodeForm(FlaskForm):
-    expiration_time = DateField('Expiration Time', format='%Y-%m-%d', validators=[Optional()])
+    expiration_time = DateField('Expiration Time',
+                                format='%Y-%m-%d',
+                                validators=[Optional(), registration_code_expiration_date_validator])
     max_usages = IntegerField('Max Usages', validators=[NumberRange(min=1)])