--- a/src/http/modules/ngx_http_not_modified_filter_module.c
+++ b/src/http/modules/ngx_http_not_modified_filter_module.c
@@ -50,7 +50,8 @@ static ngx_http_output_header_filter_pt 
 static
 ngx_int_t ngx_http_not_modified_header_filter(ngx_http_request_t *r)
 {
-    time_t  ims;
+    time_t                     ims;
+    ngx_http_core_loc_conf_t  *clcf;
 
     if (r->headers_out.status != NGX_HTTP_OK
         || r != r->main
@@ -66,17 +67,22 @@ ngx_int_t ngx_http_not_modified_header_f
     ngx_log_debug2(NGX_LOG_DEBUG_HTTP, r->connection->log, 0,
                    "http ims:%d lm:%d", ims, r->headers_out.last_modified_time);
 
-    /*
-     * I think that the equality of the dates is correcter
-     */
+    if (ims != r->headers_out.last_modified_time) {
+
+        clcf = ngx_http_get_module_loc_conf(r, ngx_http_core_module);
 
-    if (ims == r->headers_out.last_modified_time) {
-        r->headers_out.status = NGX_HTTP_NOT_MODIFIED;
-        r->headers_out.content_type.len = 0;
-        ngx_http_clear_content_length(r);
-        ngx_http_clear_accept_ranges(r);
+        if (clcf->if_modified_since == 0
+            || ims < r->headers_out.last_modified_time)
+        {
+            return ngx_http_next_header_filter(r);
+        }
     }
 
+    r->headers_out.status = NGX_HTTP_NOT_MODIFIED;
+    r->headers_out.content_type.len = 0;
+    ngx_http_clear_content_length(r);
+    ngx_http_clear_accept_ranges(r);
+
     return ngx_http_next_header_filter(r);
 }