diff --git a/include/grgsm/misc_utils/CMakeLists.txt b/include/grgsm/misc_utils/CMakeLists.txt
index 878fcfd..5bfef6a 100644
--- a/include/grgsm/misc_utils/CMakeLists.txt
+++ b/include/grgsm/misc_utils/CMakeLists.txt
@@ -34,5 +34,6 @@
     message_printer.h
     tmsi_dumper.h
     msg_to_tag.h
-    controlled_fractional_resampler_cc.h DESTINATION include/grgsm/misc_utils
+    controlled_fractional_resampler_cc.h 
+    fn_time.h DESTINATION include/grgsm/misc_utils
 )
diff --git a/include/grgsm/misc_utils/fn_time.h b/include/grgsm/misc_utils/fn_time.h
new file mode 100644
index 0000000..240506a
--- /dev/null
+++ b/include/grgsm/misc_utils/fn_time.h
@@ -0,0 +1,51 @@
+/* -*- c++ -*- */
+/* @file
+ * @author Piotr Krysik <ptrkrysik@gmail.com>
+ * @section LICENSE
+ * 
+ * Gr-gsm is free software; you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation; either version 3, or (at your option)
+ * any later version.
+ * 
+ * Gr-gsm is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ * 
+ * You should have received a copy of the GNU General Public License
+ * along with gr-gsm; see the file COPYING.  If not, write to
+ * the Free Software Foundation, Inc., 51 Franklin Street,
+ * Boston, MA 02110-1301, USA.
+ * 
+ */
+
+
+#ifndef INCLUDED_GRGSM_FN_TIME_H
+#define INCLUDED_GRGSM_FN_TIME_H
+
+#include <stdint.h>
+#include <uhd/types/time_spec.hpp>
+
+namespace gr {
+  namespace gsm {
+   
+    /**
+     * Computes difference between reference frame number
+     * and a second frame number. 
+     * @param  fn_ref    reference frame number modulo GSM_HYPER_FRAME
+     * @param  fn        second frame number modulo GSM_HYPER_FRAME
+     * @param  time_ref  precise timestamp of the first sample in the fn_ref
+     * @param  time_hint coarse time for fn that is used as a hint to avoid
+     *                   ambiguities caused by modulo operation applied to
+     *                   frame numbers
+     * @return           difference between fn_ref and fn
+     */
+    uhd::time_spec_t fn_time_delta(uint32_t fn_ref, uhd::time_spec_t time_ref, uint32_t fn_x, 
+      uhd::time_spec_t time_hint, uint32_t ts_num, uint32_t ts_ref);
+
+  } // namespace grgsm
+} // namespace gr
+
+#endif /* INCLUDED_GRGSM_FN_TIME_H */
+
diff --git a/lib/misc_utils/CMakeLists.txt b/lib/misc_utils/CMakeLists.txt
index 1c51fb6..8597553 100644
--- a/lib/misc_utils/CMakeLists.txt
+++ b/lib/misc_utils/CMakeLists.txt
@@ -32,5 +32,6 @@
     message_printer_impl.cc
     msg_to_tag_impl.cc
     tmsi_dumper_impl.cc
+    fn_time.cc
 )
 
diff --git a/lib/misc_utils/fn_time.cc b/lib/misc_utils/fn_time.cc
new file mode 100644
index 0000000..5248211
--- /dev/null
+++ b/lib/misc_utils/fn_time.cc
@@ -0,0 +1,105 @@
+/* -*- c++ -*- */
+/* @file
+ * @author Piotr Krysik <ptrkrysik@gmail.com>
+ * @author Vadim Yanitskiy <axilirator@gmail.com>
+ * @section LICENSE
+ * 
+ * Gr-gsm is free software; you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation; either version 3, or (at your option)
+ * any later version.
+ * 
+ * Gr-gsm is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ * 
+ * You should have received a copy of the GNU General Public License
+ * along with gr-gsm; see the file COPYING.  If not, write to
+ * the Free Software Foundation, Inc., 51 Franklin Street,
+ * Boston, MA 02110-1301, USA.
+ * 
+ */
+
+#include <grgsm/misc_utils/fn_time.h>
+#include <math.h>
+#include <uhd/types/time_spec.hpp>
+
+#define GSM_HYPER_FRAME 26 * 51 * 2048
+#define GSM_SYM_RATE 13.0e6 / 48.0
+
+#define GSM_TS_PERIOD 156.25 / GSM_SYM_RATE
+#define GSM_FN_PERIOD 8 * GSM_TS_PERIOD
+
+namespace gr {
+  namespace gsm {
+    using namespace uhd;
+  	/**
+  	 * Computes difference between two frame numbers modulo
+  	 * GSM_HYPER_FRAME / 2. The result is correct if difference
+  	 * between the frame numbers is not bigger than GSM_HYPER_FRAME / 2.
+  	 * @param  fn1 first frame number
+  	 * @param  fn2 second frame number
+  	 * @return     computed difference
+  	 */
+    static uint32_t fnmod_delta(uint32_t fn1, uint32_t fn2)
+    {
+      uint32_t delta, h2;
+
+      delta = (fn1 % GSM_HYPER_FRAME) - (fn2 % GSM_HYPER_FRAME);
+      h2 = GSM_HYPER_FRAME / 2;
+
+      if (delta >= h2)
+      	delta -= GSM_HYPER_FRAME;
+      else
+      	delta += GSM_HYPER_FRAME;
+
+      return delta;
+    }
+
+    static int fn_time_diff_delta(uint32_t fn, uint32_t fn_ref,
+      time_spec_t time_diff_hint)
+    {
+      int frames_diff, fn_delta;
+
+      frames_diff = int(round(time_diff_hint.get_real_secs() / GSM_FN_PERIOD));
+      fn_delta = fnmod_delta(fn, fn_ref + frames_diff) + frames_diff;
+
+      return fn_delta;
+    }
+
+    /**
+     * Computes difference between reference frame number
+     * and a second frame number. 
+     * @param  fn_ref    reference frame number modulo GSM_HYPER_FRAME
+     * @param  fn        second frame number modulo GSM_HYPER_FRAME
+     * @param  time_ref  precise timestamp of the first sample in the fn_ref
+     * @param  time_hint coarse time for fn that is used as a hint to avoid
+     *                   ambiguities caused by modulo operation applied to
+     *                   frame numbers
+     * @return           difference between fn_ref and fn
+     */
+    time_spec_t fn_time_delta(uint32_t fn_ref, time_spec_t time_ref, uint32_t fn_x, 
+      time_spec_t time_hint, uint32_t ts_num, uint32_t ts_ref)
+    {
+			time_spec_t time_diff_hint = time_hint - time_ref;
+      uint32_t fn_delta = fn_time_diff_delta(fn_x,fn_ref, time_diff_hint);
+      time_spec_t time_x_precise = fn_delta * GSM_FN_PERIOD
+        + time_ref + (ts_num - ts_ref) * GSM_TS_PERIOD;
+      
+      return time_x_precise;
+    }
+/*
+    def fn_time_delta(fn_ref, time_ref, fn_x, time_hint=None, ts_num=0, ts_ref=0):
+      if time_hint is None:
+          time_hint = time_ref
+          
+      time_diff_hint = time_hint-time_ref
+      fn_delta = fn_time_diff_delta(fn_x,fn_ref, time_diff_hint)
+      time_x_precise = fn_delta*__frame_period+time_ref+(ts_num-ts_ref)*__ts_period
+      
+      return fn_delta, time_x_precise
+*/
+  } /* namespace gsm */
+} /* namespace gr */
+
diff --git a/swig/grgsm_swig.i b/swig/grgsm_swig.i
index a67e264..08d95f1 100644
--- a/swig/grgsm_swig.i
+++ b/swig/grgsm_swig.i
@@ -65,6 +65,7 @@
 #include "grgsm/misc_utils/msg_to_tag.h"
 #include "grgsm/misc_utils/controlled_fractional_resampler_cc.h"
 #include "grgsm/trx_interface/trx.h"
+#include "grgsm/misc_utils/fn_time.h"
 %}
 
 %include "constants.i"
@@ -146,3 +147,4 @@
 GR_SWIG_BLOCK_MAGIC2(gsm, message_sink);
 %include "grgsm/trx_interface/trx.h"
 GR_SWIG_BLOCK_MAGIC2(grgsm, trx);
+%include "grgsm/misc_utils/fn_time.h"