Add a 'start_time' variable to nginx 1.2.1 to support an X-REQUEST-START header. This header is used by New Relic RPM to record queue time.

request_start_variable.patch

--- src/http/ngx_http_variables.c.orig	2012-06-07 12:47:34.000000000 -0400
+++ src/http/ngx_http_variables.c	2012-06-07 12:55:51.000000000 -0400
@@ -99,6 +99,8 @@
     ngx_http_variable_value_t *v, uintptr_t data);
 static ngx_int_t ngx_http_variable_pid(ngx_http_request_t *r,
     ngx_http_variable_value_t *v, uintptr_t data);
+static ngx_int_t ngx_http_variable_start_time(ngx_http_request_t *r,
+    ngx_http_variable_value_t *v, uintptr_t data);
 
 /*
  * TODO:
@@ -263,6 +265,9 @@
     { ngx_string("pid"), NULL, ngx_http_variable_pid,
       0, 0, 0 },
 
+    { ngx_string("start_time"), NULL, ngx_http_variable_start_time,
+      0, 0, 0 },
+
 #if (NGX_HAVE_TCP_INFO)
     { ngx_string("tcpinfo_rtt"), NULL, ngx_http_variable_tcpinfo,
       0, NGX_HTTP_VAR_NOCACHEABLE, 0 },
@@ -1796,6 +1801,27 @@
     return NGX_OK;
 }
 
+static ngx_int_t
+ngx_http_variable_start_time(ngx_http_request_t *r,
+    ngx_http_variable_value_t *v, uintptr_t data)
+{
+    u_char  *p;
+
+    p = ngx_pnalloc(r->pool, NGX_INT64_LEN);
+    if (p == NULL) {
+        return NGX_ERROR;
+    }
+
+    uint64_t usec = (((uint64_t)r->start_sec * 1000 * 1000) + ((uint64_t)r->start_msec * 1000));
+
+    v->len = ngx_sprintf(p, "%L", usec) - p;
+    v->valid = 1;
+    v->no_cacheable = 0;
+    v->not_found = 0;
+    v->data = p;
+
+    return NGX_OK;
+}
 
 void *
 ngx_http_map_find(ngx_http_request_t *r, ngx_http_map_t *map, ngx_str_t *match)

Unfortunate that there's no way to achieve this without a patch...