diff --git a/lib/stream-chat/channel.rb b/lib/stream-chat/channel.rb
index ed91ed7..19b2ec1 100644
--- a/lib/stream-chat/channel.rb
+++ b/lib/stream-chat/channel.rb
@@ -234,7 +234,9 @@ def update_member_partial(user_id, set: nil, unset: nil)
     # Adds members to the channel.
     sig { params(user_ids: T::Array[String], options: T.untyped).returns(StreamChat::StreamResponse) }
     def add_members(user_ids, **options)
-      payload = options.merge({ add_members: user_ids })
+      payload = options.dup
+      payload[:hide_history_before] = StreamChat.normalize_timestamp(payload[:hide_history_before]) if payload[:hide_history_before]
+      payload = payload.merge({ add_members: user_ids })
       update(nil, nil, **payload)
     end
 
diff --git a/lib/stream-chat/client.rb b/lib/stream-chat/client.rb
index 13a96f1..86291ae 100644
--- a/lib/stream-chat/client.rb
+++ b/lib/stream-chat/client.rb
@@ -738,7 +738,7 @@ def delete_channels(cids, hard_delete: false)
     # Revoke tokens for an application issued since the given date.
     sig { params(before: T.any(DateTime, String)).returns(StreamChat::StreamResponse) }
     def revoke_tokens(before)
-      before = before.rfc3339 if before.instance_of?(DateTime)
+      before = StreamChat.normalize_timestamp(before)
       update_app_settings({ 'revoke_tokens_issued_before' => before })
     end
 
@@ -751,7 +751,7 @@ def revoke_user_token(user_id, before)
     # Revoke tokens for users issued since.
     sig { params(user_ids: T::Array[String], before: T.any(DateTime, String)).returns(StreamChat::StreamResponse) }
     def revoke_users_token(user_ids, before)
-      before = before.rfc3339 if before.instance_of?(DateTime)
+      before = StreamChat.normalize_timestamp(before)
 
       updates = []
       user_ids.map do |user_id|
@@ -1024,7 +1024,7 @@ def query_threads(filter, sort: nil, **options)
     sig { params(message_id: String, user_id: String, remind_at: T.nilable(DateTime)).returns(StreamChat::StreamResponse) }
     def create_reminder(message_id, user_id, remind_at = nil)
       data = { user_id: user_id }
-      data[:remind_at] = remind_at.rfc3339 if remind_at.instance_of?(DateTime)
+      data[:remind_at] = StreamChat.normalize_timestamp(remind_at) if remind_at
       post("messages/#{message_id}/reminders", data: data)
     end
 
@@ -1036,7 +1036,7 @@ def create_reminder(message_id, user_id, remind_at = nil)
     sig { params(message_id: String, user_id: String, remind_at: T.nilable(DateTime)).returns(StreamChat::StreamResponse) }
     def update_reminder(message_id, user_id, remind_at = nil)
       data = { user_id: user_id }
-      data[:remind_at] = remind_at.rfc3339 if remind_at
+      data[:remind_at] = StreamChat.normalize_timestamp(remind_at) if remind_at
       patch("messages/#{message_id}/reminders", data: data)
     end
 
diff --git a/lib/stream-chat/util.rb b/lib/stream-chat/util.rb
index 4b343be..6842384 100644
--- a/lib/stream-chat/util.rb
+++ b/lib/stream-chat/util.rb
@@ -14,4 +14,14 @@ def self.get_sort_fields(sort)
     end
     sort_fields
   end
+
+  # Normalizes a timestamp to RFC 3339 / ISO 8601 string format.
+  sig { params(timestamp: T.any(DateTime, Time, String)).returns(String) }
+  def self.normalize_timestamp(timestamp)
+    case timestamp
+    when DateTime then timestamp.rfc3339
+    when Time then timestamp.iso8601
+    else timestamp
+    end
+  end
 end
diff --git a/spec/channel_spec.rb b/spec/channel_spec.rb
index 0971ef3..1215c1c 100644
--- a/spec/channel_spec.rb
+++ b/spec/channel_spec.rb
@@ -145,7 +145,8 @@ def loop_times(times)
     response = @channel.remove_members([@random_users[0][:id], @random_users[1][:id]])
     expect(response['members'].length).to eq 0
 
-    @channel.add_members([@random_users[0][:id]])
+    history_cutoff = DateTime.now - 1
+    @channel.add_members([@random_users[0][:id]], hide_history_before: history_cutoff)
     response = @channel.add_members([@random_users[1][:id]], hide_history: true)
     expect(response['members'].length).to eq 2
     response['members']&.each do |m|