diff --git a/History.md b/History.md
index ad105af4..776e6967 100644
--- a/History.md
+++ b/History.md
@@ -1,6 +1,7 @@
 ## Master
 
 * Features
+  * Instrument started_at in /stats (#1844)
   * Add log_formatter configuration (#1816)
 
 * Bugfixes
diff --git a/lib/puma/cluster.rb b/lib/puma/cluster.rb
index 492db590..658cd5c6 100644
--- a/lib/puma/cluster.rb
+++ b/lib/puma/cluster.rb
@@ -95,12 +95,13 @@ module Puma
         @signal = "TERM"
         @options = options
         @first_term_sent = nil
+        @started_at = Time.now
         @last_checkin = Time.now
         @last_status = '{}'
         @dead = false
       end
 
-      attr_reader :index, :pid, :phase, :signal, :last_checkin, :last_status
+      attr_reader :index, :pid, :phase, :signal, :last_checkin, :last_status, :started_at
 
       def booted?
         @stage == :booted
@@ -376,8 +377,8 @@ module Puma
     def stats
       old_worker_count = @workers.count { |w| w.phase != @phase }
       booted_worker_count = @workers.count { |w| w.booted? }
-      worker_status = '[' + @workers.map { |w| %Q!{ "pid": #{w.pid}, "index": #{w.index}, "phase": #{w.phase}, "booted": #{w.booted?}, "last_checkin": "#{w.last_checkin.utc.iso8601}", "last_status": #{w.last_status} }!}.join(",") + ']'
-      %Q!{ "workers": #{@workers.size}, "phase": #{@phase}, "booted_workers": #{booted_worker_count}, "old_workers": #{old_worker_count}, "worker_status": #{worker_status} }!
+      worker_status = '[' + @workers.map { |w| %Q!{ "started_at": "#{w.started_at.utc.iso8601}", "pid": #{w.pid}, "index": #{w.index}, "phase": #{w.phase}, "booted": #{w.booted?}, "last_checkin": "#{w.last_checkin.utc.iso8601}", "last_status": #{w.last_status} }!}.join(",") + ']'
+      %Q!{ "started_at": "#{@started_at.utc.iso8601}", "workers": #{@workers.size}, "phase": #{@phase}, "booted_workers": #{booted_worker_count}, "old_workers": #{old_worker_count}, "worker_status": #{worker_status} }!
     end
 
     def preload?
diff --git a/lib/puma/runner.rb b/lib/puma/runner.rb
index 213200fb..0c40dcbb 100644
--- a/lib/puma/runner.rb
+++ b/lib/puma/runner.rb
@@ -14,6 +14,7 @@ module Puma
       @options = cli.options
       @app = nil
       @control = nil
+      @started_at = Time.now
     end
 
     def daemon?
diff --git a/lib/puma/single.rb b/lib/puma/single.rb
index 3cc5e4d8..a1673a8e 100644
--- a/lib/puma/single.rb
+++ b/lib/puma/single.rb
@@ -18,7 +18,7 @@ module Puma
       r = @server.running || 0
       t = @server.pool_capacity || 0
       m = @server.max_threads || 0
-      %Q!{ "backlog": #{b}, "running": #{r}, "pool_capacity": #{t}, "max_threads": #{m} }!
+      %Q!{ "started_at": "#{@started_at.utc.iso8601}", "backlog": #{b}, "running": #{r}, "pool_capacity": #{t}, "max_threads": #{m} }!
     end
 
     def restart
diff --git a/test/test_cli.rb b/test/test_cli.rb
index 571c7999..ec280388 100644
--- a/test/test_cli.rb
+++ b/test/test_cli.rb
@@ -62,8 +62,8 @@ class TestCLI < Minitest::Test
     body = s.read
     s.close
 
-    assert_equal '{ "backlog": 0, "running": 0, "pool_capacity": 16, "max_threads": 16 }', body.split(/\r?\n/).last
-    assert_equal '{ "backlog": 0, "running": 0, "pool_capacity": 16, "max_threads": 16 }', Puma.stats
+    assert_match /{ "started_at": "\d{4}-\d{2}-\d{2}T\d{2}:\d{2}:\d{2}Z", "backlog": 0, "running": 0, "pool_capacity": 16, "max_threads": 16 }/, body.split(/\r?\n/).last
+    assert_match /{ "started_at": "\d{4}-\d{2}-\d{2}T\d{2}:\d{2}:\d{2}Z", "backlog": 0, "running": 0, "pool_capacity": 16, "max_threads": 16 }/, Puma.stats
 
   ensure
     cli.launcher.stop
@@ -101,7 +101,7 @@ class TestCLI < Minitest::Test
     s = UNIXSocket.new @tmp_path
     s << "GET /stats HTTP/1.0\r\n\r\n"
     body = s.read
-    assert_match(/\{ "workers": 2, "phase": 0, "booted_workers": 2, "old_workers": 0, "worker_status": \[\{ "pid": \d+, "index": 0, "phase": 0, "booted": true, "last_checkin": "[^"]+", "last_status": \{ "backlog":0, "running":2, "pool_capacity":2, "max_threads": 2 \} \},\{ "pid": \d+, "index": 1, "phase": 0, "booted": true, "last_checkin": "[^"]+", "last_status": \{ "backlog":0, "running":2, "pool_capacity":2, "max_threads": 2 \} \}\] \}/, body.split("\r\n").last)
+    assert_match(/\{ "started_at": "\d{4}-\d{2}-\d{2}T\d{2}:\d{2}:\d{2}Z", "workers": 2, "phase": 0, "booted_workers": 2, "old_workers": 0, "worker_status": \[\{ "started_at": "\d{4}-\d{2}-\d{2}T\d{2}:\d{2}:\d{2}Z", "pid": \d+, "index": 0, "phase": 0, "booted": true, "last_checkin": "[^"]+", "last_status": \{ "backlog":0, "running":2, "pool_capacity":2, "max_threads": 2 \} \},\{ "started_at": "\d{4}-\d{2}-\d{2}T\d{2}:\d{2}:\d{2}Z", "pid": \d+, "index": 1, "phase": 0, "booted": true, "last_checkin": "[^"]+", "last_status": \{ "backlog":0, "running":2, "pool_capacity":2, "max_threads": 2 \} \}\] \}/, body.split("\r\n").last)
 
     cli.launcher.stop
     t.join
@@ -125,7 +125,7 @@ class TestCLI < Minitest::Test
     s << "GET /stats HTTP/1.0\r\n\r\n"
     body = s.read
 
-    assert_equal '{ "backlog": 0, "running": 0, "pool_capacity": 16, "max_threads": 16 }', body.split("\r\n").last
+    assert_match /{ "started_at": "\d{4}-\d{2}-\d{2}T\d{2}:\d{2}:\d{2}Z", "backlog": 0, "running": 0, "pool_capacity": 16, "max_threads": 16 }/, body.split("\r\n").last
 
     cli.launcher.stop
     t.join