diff --git a/videocr/utils.py b/videocr/utils.py
index ee6161c..13e955f 100644
--- a/videocr/utils.py
+++ b/videocr/utils.py
@@ -1,5 +1,6 @@
 from urllib.request import urlopen
 import shutil
+import datetime
 
 from . import constants
 
@@ -19,3 +20,27 @@ def download_lang_data(lang: str):
 
             with urlopen(url) as res, open(filepath, 'w+b') as f:
                 shutil.copyfileobj(res, f)
+
+
+# convert time string to frame index
+def get_frame_index(time_str: str, fps: float):
+    t = time_str.split(':')
+    t = list(map(float, t))
+    if len(t) == 3:
+        td = datetime.timedelta(hours=t[0], minutes=t[1], seconds=t[2])
+    elif len(t) == 2:
+        td = datetime.timedelta(minutes=t[0], seconds=t[1])
+    else:
+        raise ValueError(
+            'Time data "{}" does not match format "%H:%M:%S"'.format(time_str))
+    index = int(td.total_seconds() * fps)
+    return index
+
+
+# convert frame index into SRT timestamp
+def get_srt_timestamp(frame_index: int, fps: float):
+    td = datetime.timedelta(seconds=frame_index / fps)
+    ms = td.microseconds // 1000
+    m, s = divmod(td.seconds, 60)
+    h, m = divmod(m, 60)
+    return '{:02d}:{:02d}:{:02d},{:03d}'.format(h, m, s, ms)
diff --git a/videocr/video.py b/videocr/video.py
index f79eaf7..754a17a 100644
--- a/videocr/video.py
+++ b/videocr/video.py
@@ -1,10 +1,10 @@
 from __future__ import annotations
-from multiprocessing import Pool
-import datetime
+import multiprocessing
 import pytesseract
 import cv2
 
 from . import constants
+from . import utils
 from .models import PredictedFrame, PredictedSubtitle
 from .opencv_adapter import Capture
 
@@ -27,12 +27,12 @@ class Video:
             self.height = int(v.get(cv2.CAP_PROP_FRAME_HEIGHT))
 
     def run_ocr(self, lang: str, time_start: str, time_end: str,
-                conf_threshold:int, use_fullframe: bool) -> None:
+                conf_threshold: int, use_fullframe: bool) -> None:
         self.lang = lang
         self.use_fullframe = use_fullframe
 
-        ocr_start = self._frame_index(time_start) if time_start else 0
-        ocr_end = self._frame_index(time_end) if time_end else self.num_frames
+        ocr_start = utils.get_frame_index(time_start, self.fps) if time_start else 0
+        ocr_end = utils.get_frame_index(time_end, self.fps) if time_end else self.num_frames
 
         if ocr_end < ocr_start:
             raise ValueError('time_start is later than time_end')
@@ -46,31 +46,11 @@ class Video:
             # perform ocr to frames in parallel
             it_ocr = pool.imap(self._image_to_data, frames, chunksize=10)
             self.pred_frames = [
-                PredictedFrame(i + ocr_start, data, conf_threshold) 
-                for i, data in enumerate(it_ocr)]
+                PredictedFrame(i + ocr_start, data, conf_threshold)
+                for i, data in enumerate(it_ocr)
+            ]
 
-        v.release()
-
-    # convert time str to frame index
-    def _frame_index(self, time: str) -> int:
-        t = time.split(':')
-        t = list(map(float, t))
-        if len(t) == 3:
-            td = datetime.timedelta(hours=t[0], minutes=t[1], seconds=t[2])
-        elif len(t) == 2:
-            td = datetime.timedelta(minutes=t[0], seconds=t[1])
-        else:
-            raise ValueError(
-                'time data "{}" does not match format "%H:%M:%S"'.format(time))
-
-        index = int(td.total_seconds() * self.fps)
-        if index > self.num_frames or index < 0:
-            raise ValueError(
-                'time data "{}" exceeds video duration'.format(time))
-
-        return index
-
-    def _single_frame_ocr(self, img) -> str:
+    def _image_to_data(self, img) -> str:
         if not self.use_fullframe:
             # only use bottom half of the frame by default
             img = img[self.height // 2:, :]
@@ -82,8 +62,8 @@ class Video:
         return ''.join(
             '{}\n{} --> {}\n{}\n\n'.format(
                 i,
-                self._srt_timestamp(sub.index_start),
-                self._srt_timestamp(sub.index_end),
+                utils.get_srt_timestamp(sub.index_start, self.fps),
+                utils.get_srt_timestamp(sub.index_end, self.fps),
                 sub.text)
             for i, sub in enumerate(self.pred_subs))
 
@@ -133,10 +113,3 @@ class Video:
             sub = PredictedSubtitle(ls.frames + sub.frames, sub.sim_threshold)
 
         self.pred_subs.append(sub)
-
-    def _srt_timestamp(self, frame_index: int) -> str:
-        td = datetime.timedelta(seconds=frame_index / self.fps)
-        ms = td.microseconds // 1000
-        m, s = divmod(td.seconds, 60)
-        h, m = divmod(m, 60)
-        return '{:02d}:{:02d}:{:02d},{:03d}'.format(h, m, s, ms)