diff --git a/lib/perfectsched.rb b/lib/perfectsched.rb
index 441349c..aa00e43 100644
--- a/lib/perfectsched.rb
+++ b/lib/perfectsched.rb
@@ -52,7 +52,7 @@ module PerfectSched
     require File.expand_path(v, File.dirname(__FILE__))
   }
 
-  require 'cron-spec'
+  require 'chrono'
   require 'tzinfo'
 
   def self.open(config, &block)
@@ -71,28 +71,9 @@ def self.open(config, &block)
   end
 
   def self.cron_time(cron, timestamp, timezone)
-    begin
-      tab = CronSpec::CronSpecification.new(cron)
-    rescue
-      raise ArgumentError, "invalid cron format: #{$!}: #{cron.inspect}"
-    end
-
-    begin
-      tz = TZInfo::Timezone.get(timezone) if timezone
-    rescue
-      raise ArgumentError, "unknown timezone: #{$!}: #{timezone.inspect}"
-    end
-
-    ts = (timestamp + 59) / 60 * 60
-    while true
-      t = Time.at(ts).utc
-      t = tz.utc_to_local(t) if tz
-      if tab.is_specification_in_effect?(t)
-        return ts
-      end
-      ts += 60
-      # FIXME break
-    end
+    ts = timestamp - 1 # compatibility
+    t = Time.find_zone!(timezone || 'UTC'.freeze).at(ts)
+    Chrono::NextTime.new(now: t, source: cron).to_time.to_i
   end
 
   def self.next_time(cron, before_timestamp, timezone)
diff --git a/perfectsched.gemspec b/perfectsched.gemspec
index 86f89ac..5de3951 100644
--- a/perfectsched.gemspec
+++ b/perfectsched.gemspec
@@ -16,7 +16,7 @@ Gem::Specification.new do |gem|
   gem.executables = `git ls-files -- bin/*`.split("\n").map{ |f| File.basename(f) }
   gem.require_paths = ['lib']
 
-  gem.add_dependency "cron-spec", [">= 0.1.2", "<= 0.1.2"]
+  gem.add_dependency "chrono", "~> 0.2.0"
   gem.add_dependency "sequel", "~> 3.48.0"
   gem.add_dependency "tzinfo", "~> 1.1"
   gem.add_dependency "perfectqueue", "~> 0.8.41"
diff --git a/spec/schedule_collection_spec.rb b/spec/schedule_collection_spec.rb
index 76dbcb0..34c0aed 100644
--- a/spec/schedule_collection_spec.rb
+++ b/spec/schedule_collection_spec.rb
@@ -110,11 +110,11 @@
   it 'invalid cron format' do
     expect {
       sc.add('sched01', 't01', :cron=>'???')
-    }.to raise_error ArgumentError
+    }.to raise_error StandardError
 
     expect {
       sc.add('sched01', 't01', :cron=>'* * * * * *')
-    }.to raise_error ArgumentError
+    }.to raise_error StandardError
   end
 
   it 'fail duplicated add' do