diff --git a/matemat/webserver/httpd.py b/matemat/webserver/httpd.py
index aa282e1..e962a8c 100644
--- a/matemat/webserver/httpd.py
+++ b/matemat/webserver/httpd.py
@@ -10,7 +10,6 @@ from http.server import HTTPServer, BaseHTTPRequestHandler
 from http.cookies import SimpleCookie
 from uuid import uuid4
 from datetime import datetime, timedelta
-from time import localtime
 
 import jinja2
 
@@ -349,10 +348,10 @@ class HttpHandler(BaseHTTPRequestHandler):
                 # Parse the If-Modified-Since header to check whether the browser can reuse cached content
                 datestr: str = self.headers.get('If-Modified-Since', 'Thu, 01 Jan 1970 00:00:00 GMT')
                 maxage: datetime = datetime.strptime(datestr, '%a, %d %b %Y %H:%M:%S %Z')
-                # File modification time in LOCAL time
+                # Get file modification time
                 filestat: int = int(os.path.getmtime(filepath))
-                # Create timezone-unaware datetime object and subtract UTC offset
-                fileage: datetime = datetime.fromtimestamp(filestat) - timedelta(seconds=localtime().tm_gmtoff)
+                # Create UTC datetime object from mtime
+                fileage: datetime = datetime.utcfromtimestamp(filestat)
 
                 # If the file has not been replaced by a newer version than requested by the client, send a 304 response
                 if fileage <= maxage: