1,365. One Who Neglected to Sit Shiva

216:3 When it comes to the twelve months of mourning for a parent, we don’t say that part of the day is like the whole day. Just the opposite, the practice is to add the  day of yahrzeit and to observe all the restrictions of the twelve months on that day, even if it falls on Shabbos. However, on a leap year one only observes mourning for a parent for twelve months, which end before the yahrzeit. In such a case, one does not revert to mourning.

217:1 If a mourner did not observe mourning during shiva, whether accidentally or on purpose, he can make it up at any time during the month of shloshim, except for tearing his garment; if he didn’t tear his garment at the time of grief, he may only due so during shiva, which is still considered the time of his grief. For a parent, one may tear at any point.