My sketch last a few second so the 1Hz ds1307 pulse is not useful for me.
I found a solution that I suppose most of you will consider rough. I hope it will be useful for anyone.
This is my rtc:
http://snootlab.com/shields-snootlab/86-memoire-fr.html
I loaded the sleep sketch example form here:
http://playground.arduino.cc/Learning/arduinoSleepCode
I unmount a R10 V10 diode from a broken power supply and added a 100nF capacitor and I connected them between a dig pin (pinout mode) and the pin 2 (low).
I didn't found the datasheet of the capacitor but i measured the resistance: 0.77Mohm and 73Mohm inverting the pole. I suppose it is similar to A10 V10 diode.
Two mseconds are enough to charge the capacitor. It also works connected to a pin used for other purpose.
The high resistance of the diode makes dicharge the capacitor, just to the same pin (pinout mode) that charged it, in about 50 seconds.
A little delay may be added to comparing time of ds1307.
I tried to substitute the diode with a led diode but the capacitor time discharge varied continuosly.
dig pin--------diode----------capacitor----------pin2
(I didn't manage to insert the fritzing image)