Once I uploaded the new firmware to my bedside clock, it was easy to figure out if the self-correction routine would work by comparing it with a time reference.
I elected my smartphone as the reference, but in a couple of days I noticed that the homemade clock was gaining time, much more than the expected 2"/day.
Well, it turned out that my smartphone clock was not properly adjusted. The setting "sync time with network" was unchecked. Not a big deal, if the smartphone wasnt't loosing time itself on a daily basis!
So, if you're trying to calibrate a cheap DS1302/DS1307 module, take long time measurements (one/two months) and make sure your reference is reliably in sync with a reliable source. :)
And, probably most important, don't do this serious stuff before going to bed, after a day full of the usual routine, family, work, stress, ...