diff --git a/bin/is b/bin/is
index 3f05cf357c2a2c0cd5cb96788016b6d02055b039..2186a0ddfe206d1beed2eaf8c17bbe38966c6e4c 100755
--- a/bin/is
+++ b/bin/is
@@ -21,7 +21,6 @@ InstallSystems Command line Tool
 '''
 
 import os
-import time
 import datetime
 import re
 import fnmatch
@@ -127,16 +126,11 @@ def c_build(args):
             os.chdir(path)
             path = "."
         arrowlevel(1)
-        # build start time
-        t0 = time.time()
         # load source image
         simg = SourceImage(path)
         # do the job
-        simg.build(force=args.force, force_payload=args.payload,
-                   check=not args.no_check, script=not args.no_script)
-        # compute building time
-        t1 = time.time()
-        dt = int(t1 - t0)
+        dt = simg.build(force=args.force, force_payload=args.payload,
+                        check=not args.no_check, script=not args.no_script)
         arrow(u"Build time: %s" % datetime.timedelta(seconds=dt))
         if args.chdir:
             os.chdir(cwd)
diff --git a/installsystems/image.py b/installsystems/image.py
index 0e0edbc7b213f8f7079ac7de4843f34e4c997fe3..e68399fa0622274348943389de20495c1090e3d7 100644
--- a/installsystems/image.py
+++ b/installsystems/image.py
@@ -301,6 +301,8 @@ class SourceImage(Image):
         # check if free to create script tarball
         if os.path.exists(self.image_name) and force == False:
             raise ISError("Tarball already exists. Remove it before")
+        # register start time
+        t0 = time.time()
         # check python scripts
         if check:
             for d in (self.build_path, self.parser_path, self.setup_path):
@@ -322,6 +324,8 @@ class SourceImage(Image):
         jdesc = self.generate_json_description()
         # creating scripts tarball
         self.create_image(jdesc)
+        # compute building time
+        return int(time.time() - t0)
 
     def create_image(self, jdescription):
         '''