use DateTimeOffset for PremiumSince, follow conventions from other prop

src/Discord.Net.Core/Entities/Users/IGuildUser.cs

@@ -51,9 +51,9 @@ namespace Discord
         ///     Gets the date and time for when this user's guild boost began.
         /// </summary>
         /// <returns>
-        ///     A <see cref="DateTime"/> for when the user began boosting this guild; <c>null</c> if they are not boosting the guild.
+        ///     A <see cref="DateTimeOffset"/> for when the user began boosting this guild; <c>null</c> if they are not boosting the guild.
         /// </returns>
-        DateTime? PremiumSince { get; }
+        DateTimeOffset? PremiumSince { get; }
         /// <summary>
         ///     Gets a collection of IDs for the roles that this user currently possesses in the guild.
         /// </summary>

src/Discord.Net.Rest/API/Common/GuildMember.cs

@@ -18,7 +18,7 @@ namespace Discord.API
         public Optional<bool> Deaf { get; set; }
         [JsonProperty("mute")]
         public Optional<bool> Mute { get; set; }
-        [JsonProperty("premium_guild_since")]
-        public Optional<DateTime> PremiumSince { get; set; }
+        [JsonProperty("premium_since")]
+        public Optional<DateTimeOffset?> PremiumSince { get; set; }
     }
 }

src/Discord.Net.Rest/Entities/Users/RestGuildUser.cs

@@ -14,6 +14,7 @@ namespace Discord.Rest
     [DebuggerDisplay(@"{DebuggerDisplay,nq}")]
     public class RestGuildUser : RestUser, IGuildUser
     {
+        private long? _premiumSinceTicks;
         private long? _joinedAtTicks;
         private ImmutableArray<ulong> _roleIds;
 
@@ -25,7 +26,7 @@ namespace Discord.Rest
         /// <inheritdoc />
         public bool IsMuted { get; private set; }
         /// <inheritdoc />
-        public DateTime? PremiumSince { get; private set; }
+        public DateTimeOffset? PremiumSince => DateTimeUtils.FromTicks(_premiumSinceTicks);
         /// <inheritdoc />
         public ulong GuildId => Guild.Id;
 
@@ -71,7 +72,7 @@ namespace Discord.Rest
             if (model.Roles.IsSpecified)
                 UpdateRoles(model.Roles.Value);
             if (model.PremiumSince.IsSpecified)
-                PremiumSince = model.PremiumSince.Value;
+                _premiumSinceTicks = model.PremiumSince.Value?.UtcTicks;
         }
         private void UpdateRoles(ulong[] roleIds)
         {

src/Discord.Net.Rest/Entities/Users/RestWebhookUser.cs

@@ -14,7 +14,7 @@ namespace Discord.Rest
         public ulong WebhookId { get; }
         internal IGuild Guild { get; }
         /// <inheritdoc />
-        public DateTime? PremiumSince { get; private set; }
+        public DateTimeOffset? PremiumSince { get; private set; }
 
         /// <inheritdoc />
         public override bool IsWebhook => true;

src/Discord.Net.WebSocket/Entities/Users/SocketGuildUser.cs

@@ -18,6 +18,7 @@ namespace Discord.WebSocket
     [DebuggerDisplay(@"{DebuggerDisplay,nq}")]
     public class SocketGuildUser : SocketUser, IGuildUser
     {
+        private long? _premiumSinceTicks;
         private long? _joinedAtTicks;
         private ImmutableArray<ulong> _roleIds;
 
@@ -75,9 +76,8 @@ namespace Discord.WebSocket
         /// </returns>
         public SocketVoiceState? VoiceState => Guild.GetVoiceState(Id);
         public AudioInStream AudioStream => Guild.GetAudioStream(Id);
-
         /// <inheritdoc />
-        public DateTime? PremiumSince { get; private set; }
+        public DateTimeOffset? PremiumSince => DateTimeUtils.FromTicks(_premiumSinceTicks);
 
         /// <summary>
         ///     Returns the position of the user within the role hierarchy.
@@ -138,6 +138,8 @@ namespace Discord.WebSocket
                 Nickname = model.Nick.Value;
             if (model.Roles.IsSpecified)
                 UpdateRoles(model.Roles.Value);
+            if (model.PremiumSince.IsSpecified)
+                _premiumSinceTicks = model.PremiumSince.Value?.UtcTicks;
         }
         internal void Update(ClientState state, PresenceModel model, bool updatePresence)
         {

src/Discord.Net.WebSocket/Entities/Users/SocketWebhookUser.cs

@@ -63,7 +63,7 @@ namespace Discord.WebSocket
         /// <inheritdoc />
         string IGuildUser.Nickname => null;
         /// <inheritdoc />
-        DateTime? IGuildUser.PremiumSince => null;
+        DateTimeOffset? IGuildUser.PremiumSince => null;
         /// <inheritdoc />
         GuildPermissions IGuildUser.GuildPermissions => GuildPermissions.Webhook;