--- 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);
 }
 

--- a/src/http/ngx_http_core_module.c
+++ b/src/http/ngx_http_core_module.c
@@ -108,6 +108,13 @@ static ngx_conf_enum_t  ngx_http_core_sa
 };
 
 
+static ngx_conf_enum_t  ngx_http_core_if_modified_since[] = {
+    { ngx_string("exact"), 0 },
+    { ngx_string("before"), 1 },
+    { ngx_null_string, 0 }
+};
+
+
 #if (NGX_HTTP_GZIP)
 
 static ngx_conf_enum_t  ngx_http_gzip_http_version[] = {
@@ -515,6 +522,13 @@ static ngx_command_t  ngx_http_core_comm
       offsetof(ngx_http_core_loc_conf_t, server_tokens),
       NULL },
 
+    { ngx_string("if_modified_since"),
+      NGX_HTTP_MAIN_CONF|NGX_HTTP_SRV_CONF|NGX_HTTP_LOC_CONF|NGX_CONF_TAKE1,
+      ngx_conf_set_enum_slot,
+      NGX_HTTP_LOC_CONF_OFFSET,
+      offsetof(ngx_http_core_loc_conf_t, if_modified_since),
+      &ngx_http_core_if_modified_since },
+
     { ngx_string("error_page"),
       NGX_HTTP_MAIN_CONF|NGX_HTTP_SRV_CONF|NGX_HTTP_LOC_CONF|NGX_HTTP_LIF_CONF
                         |NGX_CONF_2MORE,
@@ -2663,6 +2677,7 @@ ngx_http_core_create_loc_conf(ngx_conf_t
     lcf->client_body_buffer_size = NGX_CONF_UNSET_SIZE;
     lcf->client_body_timeout = NGX_CONF_UNSET_MSEC;
     lcf->satisfy = NGX_CONF_UNSET_UINT;
+    lcf->if_modified_since = NGX_CONF_UNSET_UINT;
     lcf->internal = NGX_CONF_UNSET;
     lcf->client_body_in_file_only = NGX_CONF_UNSET;
     lcf->sendfile = NGX_CONF_UNSET;
@@ -2852,6 +2867,8 @@ ngx_http_core_merge_loc_conf(ngx_conf_t 
 
     ngx_conf_merge_uint_value(conf->satisfy, prev->satisfy,
                               NGX_HTTP_SATISFY_ALL);
+    ngx_conf_merge_uint_value(conf->if_modified_since, prev->if_modified_since,
+                              0);
     ngx_conf_merge_value(conf->internal, prev->internal, 0);
     ngx_conf_merge_value(conf->client_body_in_file_only,
                               prev->client_body_in_file_only, 0);

--- a/src/http/ngx_http_core_module.h
+++ b/src/http/ngx_http_core_module.h
@@ -299,6 +299,7 @@ struct ngx_http_core_loc_conf_s {
     time_t        keepalive_header;        /* keepalive_timeout */
 
     ngx_uint_t    satisfy;                 /* satisfy */
+    ngx_uint_t    if_modified_since;       /* if_modified_since */
 
     ngx_flag_t    internal;                /* internal */
     ngx_flag_t    client_body_in_file_only; /* client_body_in_file_only */