fix subs

Plugins/BTCPayServer.Plugins.Subscriptions/SubscriptionAppSettings.cs

@@ -1,6 +1,9 @@
-using System.Collections.Generic;
+using System;
+using System.Collections.Generic;
 using BTCPayServer.JsonConverters;
+using Microsoft.EntityFrameworkCore.Storage.ValueConversion.Internal;
 using Newtonsoft.Json;
+using Newtonsoft.Json.Converters;
 
 namespace BTCPayServer.Plugins.Subscriptions;
 
@@ -8,10 +11,48 @@ public class SubscriptionAppSettings
 {
     [JsonIgnore] public string SubscriptionName { get; set; }
     public string Description { get; set; }
-    public int DurationDays { get; set; }
+    public int Duration { get; set; }
+    
+    [JsonConverter(typeof(StringEnumConverter))]
+    public DurationType DurationType { get; set; }
     public string? FormId { get; set; }
     [JsonConverter(typeof(NumericStringJsonConverter))]
     public decimal Price { get; set; }
     public string Currency { get; set; }
     public Dictionary<string, Subscription> Subscriptions { get; set; } = new();
+    
+}
+
+public static class SubscriptionAppSettingsExtensions
+{
+    
+    public static string GetSubscriptionHumanReadableLength(this SubscriptionAppSettings settings)
+    {
+        return settings.DurationType switch
+        {
+            DurationType.Day => $"{settings.Duration} day{(settings.Duration > 1 ? "s" : "")}",
+            DurationType.Month => $"{settings.Duration} month{(settings.Duration > 1 ? "s" : "")}",
+            _ => throw new ArgumentOutOfRangeException()
+        };
+    }
+    
+    public static DateTimeOffset GetNextRenewalDate(this SubscriptionAppSettings settings, DateTimeOffset? lastRenewalDate)
+    {
+        if (lastRenewalDate == null)
+        {
+            return DateTimeOffset.UtcNow;
+        }
+        return settings.DurationType switch
+        {
+            DurationType.Day => lastRenewalDate.Value.AddDays(settings.Duration),
+            DurationType.Month => lastRenewalDate.Value.AddMonths(settings.Duration),
+            _ => throw new ArgumentOutOfRangeException()
+        };
+    }
+}
+
+public enum DurationType
+{
+    Day,
+    Month
 }