diff --git a/AppService.Acmebot/Functions/SharedActivity.cs b/AppService.Acmebot/Functions/SharedActivity.cs
index a630b53..85a249e 100644
--- a/AppService.Acmebot/Functions/SharedActivity.cs
+++ b/AppService.Acmebot/Functions/SharedActivity.cs
@@ -105,7 +105,7 @@ public async Task<IReadOnlyList<Certificate>> GetExpiringCertificates([ActivityT
             var certificates = await _webSiteManagementClient.Certificates.ListAllAsync();
 
             return certificates.Where(x => x.TagsFilter(IssuerName, _options.Endpoint))
-                               .Where(x => (x.ExpirationDate.Value - currentDateTime).TotalDays <= 30)
+                               .Where(x => (x.ExpirationDate.Value - currentDateTime).TotalDays <= _options.RenewBeforeExpiry)
                                .ToArray();
         }
 
diff --git a/AppService.Acmebot/Options/AcmebotOptions.cs b/AppService.Acmebot/Options/AcmebotOptions.cs
index 944f9d2..9eef4b6 100644
--- a/AppService.Acmebot/Options/AcmebotOptions.cs
+++ b/AppService.Acmebot/Options/AcmebotOptions.cs
@@ -5,6 +5,7 @@ namespace AppService.Acmebot.Options
     public class AcmebotOptions
     {
         [Required]
+        [Url]
         public string Endpoint { get; set; } = "https://acme-v02.api.letsencrypt.org/";
 
         [Required]
@@ -21,6 +22,9 @@ public class AcmebotOptions
 
         public string PreferredChain { get; set; }
 
+        [Range(0, 365)]
+        public int RenewBeforeExpiry { get; set; } = 30;
+
         public ExternalAccountBindingOptions ExternalAccountBinding { get; set; }
     }
 }