diff --git a/lib/vmpooler/api/v1.rb b/lib/vmpooler/api/v1.rb
index e7f6729..9915a60 100644
--- a/lib/vmpooler/api/v1.rb
+++ b/lib/vmpooler/api/v1.rb
@@ -646,8 +646,8 @@ module Vmpooler
         elsif rdata['checkout']
           result[params[:hostname]]['running'] = ((Time.now - Time.parse(rdata['checkout'])) / 60 / 60).round(2)
           result[params[:hostname]]['remaining'] = ((Time.parse(rdata['checkout']) + rdata['lifetime'].to_i*60*60 - Time.now) / 60 / 60).round(2)
-          result[params[:hostname]]['start_time'] = Time.parse(rdata['checkout'])
-          result[params[:hostname]]['end_time'] = Time.parse(rdata['checkout']) + rdata['lifetime'].to_i*60*60
+          result[params[:hostname]]['start_time'] = Time.parse(rdata['checkout']).to_datetime.rfc3339
+          result[params[:hostname]]['end_time'] = (Time.parse(rdata['checkout']) + rdata['lifetime'].to_i*60*60).to_datetime.rfc3339
           result[params[:hostname]]['state'] = 'running'
         elsif rdata['check']
           result[params[:hostname]]['state'] = 'ready'
diff --git a/spec/integration/api/v1/vm_spec.rb b/spec/integration/api/v1/vm_spec.rb
index 8134cba..5399370 100644
--- a/spec/integration/api/v1/vm_spec.rb
+++ b/spec/integration/api/v1/vm_spec.rb
@@ -50,8 +50,8 @@ describe Vmpooler::API::V1 do
         expect(response_body["lifetime"]).to eq(0)
         expect(response_body["running"]).to be >= 0
         expect(response_body["remaining"]).to be <= 0
-        expect(response_body["start_time"]).to eq("#{current_time}")
-        expect(response_body["end_time"]).to eq("#{current_time}")
+        expect(response_body["start_time"]).to eq(current_time.to_datetime.rfc3339)
+        expect(response_body["end_time"]).to eq(current_time.to_datetime.rfc3339)
         expect(response_body["state"]).to eq("running")
         expect(response_body["ip"]).to eq("")
       end