diff --git a/README.md b/README.md
index f1a761c..7611af1 100644
--- a/README.md
+++ b/README.md
@@ -39,3 +39,27 @@ make
 ## Usage
 
 For comprehensive help, use `dooked --help`
+
+### History tracking
+
+When a previous JSON result is used as input, dooked now keeps DNS records that
+were seen in earlier runs and annotates each record with:
+
+- `first-seen`: UTC time the record first appeared in the dataset
+- `last-seen`: UTC time the record was most recently observed
+- `seen`: number of runs where the record was observed
+
+This helps with load-balanced targets where valid records rotate between runs.
+The following flags surface the history:
+
+```sh
+dooked -i previous.json --fs
+dooked -i previous.json --ls 2
+dooked -i previous.json --lsd 05/31/2026
+```
+
+- `--fs` reports records first seen during the current run.
+- `--ls <days>` reports records missing from the current run and not seen for
+  at least that many days.
+- `--lsd <MM/DD/YYYY>` reports records missing from the current run and last
+  seen on or before that date.
diff --git a/dooked/include/cli_preprocessor.hpp b/dooked/include/cli_preprocessor.hpp
index 43fa1ba..2f1d5a6 100644
--- a/dooked/include/cli_preprocessor.hpp
+++ b/dooked/include/cli_preprocessor.hpp
@@ -19,12 +19,15 @@ struct cli_args_t {
   std::string resolver_filename{};
   std::string output_filename{};
   std::string input_filename{};
+  std::string last_seen_date{};
 
   int file_type{};
   int post_http_request{};
   int thread_count{};
   int content_length{-1};
+  int last_seen_days{-1};
   bool include_date{false};
+  bool first_seen_alert{false};
 };
 
 struct runtime_args_t {
@@ -36,6 +39,9 @@ struct runtime_args_t {
   http_process_e http_request_time_{};
   int thread_count{};
   int content_length{-1};
+  int last_seen_days{-1};
+  std::string last_seen_date{};
+  bool first_seen_alert{false};
 };
 
 void run_program(cli_args_t const &cli_args);
diff --git a/dooked/include/utils/io_utils.hpp b/dooked/include/utils/io_utils.hpp
index 829b09e..1a6e0d8 100644
--- a/dooked/include/utils/io_utils.hpp
+++ b/dooked/include/utils/io_utils.hpp
@@ -26,9 +26,12 @@ void trim(std::string &);
 struct json_data_t {
   std::string domain_name{};
   std::string rdata{};
+  std::string first_seen{};
+  std::string last_seen{};
   int ttl{};
   int http_code{};
   int content_length{};
+  int seen{};
   dns_record_type_e type{};
 
   static json_data_t serialize(std::string const &d, int const len,
@@ -40,6 +43,19 @@ struct json_data_t {
         dns_str_to_record_type(json_object["type"].get<json::string_t>());
     data.rdata = json_object["info"].get<json::string_t>();
     data.ttl = json_object["ttl"].get<json::number_integer_t>();
+    if (json_object.count("first-seen") != 0) {
+      data.first_seen = json_object["first-seen"].get<json::string_t>();
+    } else if (json_object.count("first_seen") != 0) {
+      data.first_seen = json_object["first_seen"].get<json::string_t>();
+    }
+    if (json_object.count("last-seen") != 0) {
+      data.last_seen = json_object["last-seen"].get<json::string_t>();
+    } else if (json_object.count("last_seen") != 0) {
+      data.last_seen = json_object["last_seen"].get<json::string_t>();
+    }
+    if (json_object.count("seen") != 0) {
+      data.seen = json_object["seen"].get<json::number_integer_t>();
+    }
     data.content_length = len;
     data.http_code = http_code;
     return data;
diff --git a/dooked/include/utils/probe_result.hpp b/dooked/include/utils/probe_result.hpp
index 07211c6..41ef435 100644
--- a/dooked/include/utils/probe_result.hpp
+++ b/dooked/include/utils/probe_result.hpp
@@ -10,8 +10,11 @@ bool case_insensitive_compare(std::string const &, std::string const &);
 
 struct probe_result_t {
   std::string rdata{};
+  std::string first_seen{};
+  std::string last_seen{};
   dns_record_type_e type{}; // RR TYPE (2 octets)
   std::uint32_t ttl{};      // time to live(4 octets)
+  int seen{0};
 
   friend bool operator==(probe_result_t const &a, probe_result_t const &b) {
     return case_insensitive_compare(a.rdata, b.rdata) && (a.type == b.type);
diff --git a/dooked/include/utils/random_utils.hpp b/dooked/include/utils/random_utils.hpp
index 0e87be2..88c2f69 100644
--- a/dooked/include/utils/random_utils.hpp
+++ b/dooked/include/utils/random_utils.hpp
@@ -1,6 +1,7 @@
 #pragma once
 
 #include <array>
+#include <cstdint>
 #include <string>
 
 namespace dooked {
diff --git a/dooked/source/cli_preprocessor.cpp b/dooked/source/cli_preprocessor.cpp
index c08d7fb..04ca005 100644
--- a/dooked/source/cli_preprocessor.cpp
+++ b/dooked/source/cli_preprocessor.cpp
@@ -4,10 +4,14 @@
 #include "utils/exceptions.hpp"
 #include "utils/random_utils.hpp"
 #include "utils/string_utils.hpp"
+#include <algorithm>
 #include <boost/asio/io_context.hpp>
 #include <boost/asio/thread_pool.hpp>
+#include <ctime>
+#include <iomanip>
 #include <set>
 #include <spdlog/spdlog.h>
+#include <sstream>
 
 // defined (and assigned to) in main.cpp
 extern bool silent;
@@ -18,6 +22,188 @@ namespace dooked {
 namespace net = boost::asio;
 using namespace fmt::v7::literals;
 
+std::string utc_time_to_string(std::time_t const timestamp) {
+  std::tm tm{};
+#ifdef _WIN32
+  gmtime_s(&tm, &timestamp);
+#else
+  gmtime_r(&timestamp, &tm);
+#endif
+  std::ostringstream out{};
+  out << std::put_time(&tm, "%Y-%m-%dT%H:%M:%SZ");
+  return out.str();
+}
+
+std::optional<std::time_t> parse_utc_time(std::string const &value) {
+  if (value.empty()) {
+    return std::nullopt;
+  }
+  std::tm tm{};
+  std::istringstream in{value};
+  in >> std::get_time(&tm, "%Y-%m-%dT%H:%M:%SZ");
+  if (in.fail()) {
+    return std::nullopt;
+  }
+#ifdef _WIN32
+  return _mkgmtime(&tm);
+#else
+  return timegm(&tm);
+#endif
+}
+
+std::optional<std::time_t> parse_us_date(std::string const &value) {
+  if (value.empty()) {
+    return std::nullopt;
+  }
+
+  for (auto const *format : {"%m/%d/%Y", "%m/%d/%Y %H:%M:%S"}) {
+    std::tm tm{};
+    std::istringstream in{value};
+    in >> std::get_time(&tm, format);
+    if (!in.fail()) {
+#ifdef _WIN32
+      return _mkgmtime(&tm);
+#else
+      return timegm(&tm);
+#endif
+    }
+  }
+  return std::nullopt;
+}
+
+bool same_record(std::string const &domain_name, probe_result_t const &record,
+                 json_data_t const &previous) {
+  return domain_name == previous.domain_name && record.type == previous.type &&
+         case_insensitive_compare(record.rdata, previous.rdata);
+}
+
+bool current_result_contains(map_container_t<probe_result_t> const &result_map,
+                             json_data_t const &previous) {
+  auto const &current_map = result_map.cresult();
+  auto const current_domain_iter = current_map.find(previous.domain_name);
+  if (current_domain_iter == current_map.end()) {
+    return false;
+  }
+  auto const &records = current_domain_iter->second.dns_result_list_;
+  return std::any_of(records.cbegin(), records.cend(),
+                     [&previous](auto const &record) {
+                       return record.type == previous.type &&
+                              case_insensitive_compare(record.rdata,
+                                                       previous.rdata);
+                     });
+}
+
+probe_result_t preserved_record(json_data_t const &previous,
+                                std::string const &now) {
+  probe_result_t record{};
+  record.rdata = previous.rdata;
+  record.first_seen = previous.first_seen.empty() ? now : previous.first_seen;
+  record.last_seen = previous.last_seen.empty() ? now : previous.last_seen;
+  record.type = previous.type;
+  record.ttl = (std::uint32_t)previous.ttl;
+  record.seen = previous.seen > 0 ? previous.seen : 1;
+  return record;
+}
+
+void report_last_seen_alerts(std::vector<json_data_t> const &previous_result,
+                             map_container_t<probe_result_t> const &result_map,
+                             runtime_args_t const &rt_args,
+                             std::time_t const now_time) {
+  if (rt_args.last_seen_days < 0 && rt_args.last_seen_date.empty()) {
+    return;
+  }
+
+  std::optional<std::time_t> threshold{};
+  if (rt_args.last_seen_days >= 0) {
+    threshold = now_time - ((std::time_t)rt_args.last_seen_days * 24 * 60 * 60);
+  } else {
+    threshold = parse_us_date(rt_args.last_seen_date);
+    if (!threshold) {
+      spdlog::warn("unable to parse --lsd date `{}`; expected MM/DD/YYYY",
+                   rt_args.last_seen_date);
+      return;
+    }
+  }
+
+  for (auto const &previous : previous_result) {
+    if (current_result_contains(result_map, previous)) {
+      continue;
+    }
+    auto const last_seen =
+        parse_utc_time(previous.last_seen).value_or(now_time);
+    if (last_seen <= *threshold) {
+      spdlog::warn("[LAST-SEEN][{}][{}] `{}` last seen {}", previous.domain_name,
+                   dns_record_type_to_str(previous.type), previous.rdata,
+                   previous.last_seen.empty() ? "unknown" : previous.last_seen);
+    }
+  }
+}
+
+void apply_history_metadata(std::vector<json_data_t> const &previous_result,
+                            map_container_t<probe_result_t> &result_map,
+                            runtime_args_t const &rt_args,
+                            std::time_t const now_time) {
+  auto const now = utc_time_to_string(now_time);
+
+  for (auto &result_pair : result_map.result()) {
+    auto const &domain_name = result_pair.first;
+    for (auto &record : result_pair.second.dns_result_list_) {
+      auto const previous_iter =
+          std::find_if(previous_result.cbegin(), previous_result.cend(),
+                       [&domain_name, &record](auto const &previous) {
+                         return same_record(domain_name, record, previous);
+                       });
+      if (previous_iter != previous_result.cend()) {
+        record.first_seen = previous_iter->first_seen.empty()
+                                ? now
+                                : previous_iter->first_seen;
+        record.last_seen = now;
+        record.seen = (previous_iter->seen > 0 ? previous_iter->seen : 1) + 1;
+      } else {
+        record.first_seen = now;
+        record.last_seen = now;
+        record.seen = 1;
+        if (rt_args.first_seen_alert) {
+          spdlog::info("[FIRST-SEEN][{}][{}] `{}`", domain_name,
+                       dns_record_type_to_str(record.type), record.rdata);
+        }
+      }
+    }
+  }
+
+  for (auto const &previous : previous_result) {
+    if (current_result_contains(result_map, previous)) {
+      continue;
+    }
+    bool const domain_exists =
+        result_map.cresult().find(previous.domain_name) !=
+        result_map.cresult().end();
+    result_map.append(previous.domain_name, preserved_record(previous, now));
+    if (!domain_exists) {
+      result_map.insert(previous.domain_name, previous.content_length,
+                        previous.http_code);
+    }
+  }
+}
+
+void initialize_history_metadata(map_container_t<probe_result_t> &result_map,
+                                 runtime_args_t const &rt_args,
+                                 std::time_t const now_time) {
+  auto const now = utc_time_to_string(now_time);
+  for (auto &result_pair : result_map.result()) {
+    auto const &domain_name = result_pair.first;
+    for (auto &record : result_pair.second.dns_result_list_) {
+      record.first_seen = now;
+      record.last_seen = now;
+      record.seen = 1;
+      if (rt_args.first_seen_alert) {
+        spdlog::info("[FIRST-SEEN][{}][{}] `{}`", domain_name,
+                     dns_record_type_to_str(record.type), record.rdata);
+      }
+    }
+  }
+}
+
 void compare_http_result(int const base_cl, json_data_t const &prev_http_result,
                          http_response_t const &current_result) {
   auto const current_req_cl = current_result.content_length_;
@@ -350,10 +536,7 @@ void start_name_checking(runtime_args_t &&rt_args) {
     }
     thread_pool->join();
   }
-  if (!silent) {
-    spdlog::info("Writing JSON output");
-  }
-  write_json_result(result_map, rt_args);
+  auto const now = std::time(nullptr);
 
   // compare old with new result -- only if we had previous record
   if (rt_args.previous_data) {
@@ -373,9 +556,18 @@ void start_name_checking(runtime_args_t &&rt_args) {
                   return std::tie(a.type, a.rdata) < std::tie(b.type, b.rdata);
                 });
     }
-    return compare_results(*rt_args.previous_data, result_map,
-                           rt_args.content_length);
+    compare_results(*rt_args.previous_data, result_map,
+                    rt_args.content_length);
+    report_last_seen_alerts(previous_data, result_map, rt_args, now);
+    apply_history_metadata(previous_data, result_map, rt_args, now);
+  } else {
+    initialize_history_metadata(result_map, rt_args, now);
   }
+
+  if (!silent) {
+    spdlog::info("Writing JSON output");
+  }
+  write_json_result(result_map, rt_args);
 }
 
 void run_program(cli_args_t const &cli_args) {
@@ -477,6 +669,9 @@ void run_program(cli_args_t const &cli_args) {
       static_cast<http_process_e>(cli_args.post_http_request);
   rt_args.thread_count = cli_args.thread_count;
   rt_args.content_length = cli_args.content_length;
+  rt_args.first_seen_alert = cli_args.first_seen_alert;
+  rt_args.last_seen_days = cli_args.last_seen_days;
+  rt_args.last_seen_date = cli_args.last_seen_date;
   return start_name_checking(std::move(rt_args));
 }
 
diff --git a/dooked/source/http/requests_handler.cpp b/dooked/source/http/requests_handler.cpp
index d21a592..322c3c9 100644
--- a/dooked/source/http/requests_handler.cpp
+++ b/dooked/source/http/requests_handler.cpp
@@ -10,6 +10,10 @@ extern bool silent;
 
 namespace dooked {
 
+std::string header_value_to_string(beast::string_view const value) {
+  return {value.data(), value.size()};
+}
+
 http_request_handler_t::http_request_handler_t(net::io_context &io_context,
                                                std::string domain_name)
     : io_{io_context}, domain_{std::move(domain_name)} {}
@@ -139,7 +143,8 @@ void http_request_handler_t::on_data_received(
   if (status_code_simple == 2) {
     response_int = response_type_e::ok;
   } else if (status_code_simple == 3) { // redirected
-    response_string = (*response_)[http::field::location].to_string();
+    response_string =
+        header_value_to_string((*response_)[http::field::location]);
     if (response_string.empty()) {
       response_int = response_type_e::unknown_response;
     } else {
@@ -171,7 +176,8 @@ void http_request_handler_t::on_data_received(
   int content_length{};
   if (response_->has_content_length()) {
     try {
-      auto const cl_str = (*response_)[http::field::content_length].to_string();
+      auto const cl_str =
+          header_value_to_string((*response_)[http::field::content_length]);
       content_length = std::stoi(cl_str);
     } catch (std::exception const &) {
     }
@@ -365,7 +371,8 @@ void https_request_handler_t::on_data_received(
   if (status_code_simple == 2) {
     response_int = response_type_e::ok;
   } else if (status_code_simple == 3) { // redirected
-    response_string = (*response_)[http::field::location].to_string();
+    response_string =
+        header_value_to_string((*response_)[http::field::location]);
     if (response_string.empty()) {
       response_int = response_type_e::unknown_response;
     } else {
@@ -392,7 +399,8 @@ void https_request_handler_t::on_data_received(
   int content_length = 0;
   if (response_->has_content_length()) {
     try {
-      auto const cl_str = (*response_)[http::field::content_length].to_string();
+      auto const cl_str =
+          header_value_to_string((*response_)[http::field::content_length]);
       content_length = std::stoi(cl_str);
     } catch (std::exception const &) {
     }
diff --git a/dooked/source/main.cpp b/dooked/source/main.cpp
index cf29460..cf92098 100644
--- a/dooked/source/main.cpp
+++ b/dooked/source/main.cpp
@@ -41,6 +41,14 @@ int main(int argc, char **argv) {
       "defers http request until after all DNS requests have been completed");
   app.add_flag("--compare-cl", compare_cl,
                "compare content-length of HTTP requests");
+  app.add_flag("--fs", cli_args.first_seen_alert,
+               "show records first seen during this run");
+  app.add_option("--ls", cli_args.last_seen_days,
+                 "show records missing from this run and not seen for this "
+                 "many days");
+  app.add_option("--lsd", cli_args.last_seen_date,
+                 "show records missing from this run and not seen since a US "
+                 "date such as MM/DD/YYYY");
 
   app.add_flag("--nbc", no_bytes_count,
                "in case `content-length` is missing in an HTTP header field,"
diff --git a/dooked/source/utils/io_utils.cpp b/dooked/source/utils/io_utils.cpp
index a1bd5d3..01541d5 100644
--- a/dooked/source/utils/io_utils.cpp
+++ b/dooked/source/utils/io_utils.cpp
@@ -6,6 +6,15 @@ void to_json(json &j, probe_result_t const &record) {
   j = json{{"ttl", record.ttl},
            {"type", dns_record_type_to_str(record.type)},
            {"info", record.rdata}};
+  if (!record.first_seen.empty()) {
+    j["first-seen"] = record.first_seen;
+  }
+  if (!record.last_seen.empty()) {
+    j["last-seen"] = record.last_seen;
+  }
+  if (record.seen > 0) {
+    j["seen"] = record.seen;
+  }
 }
 
 bool is_text_file(std::string const &file_extension) {