On a yartzeit, close family members recite kaddish. Some minyanim have an opportunity to say kaddish d'rabbanan after (sometimes short) Torah study between mincha and maariv. Should the mourner join this kaddish d'rabbanan at the beginning of the yartzeit day (before the maariv where kaddish yatom is said)? What about at the end of the day (after the mincha where kaddish yatom is said)? Is the decision based on whether it is before or after shkia at the time or some other criteria?
This question pertains only to minyanim where the custom is for all mourners to say kaddish d'rabbanan together, not where only one person (typically the chazan or someone who has just finished conducting the Torah study) says it regardless of mourning status. For purposes of this question, please assume the mourner in question has neither of these roles.