2,880. Overlapping Nezirus Periods

Hilchos Nezirus 4:4

Pursuant to the previous halacha, let’s say that someone says, “I will be a nazir when a son is born to me, plus I’m a nazir for 100 days.” He starts observing nezirus and then a son is born to him. If 30 or more of the 100 days remain, he doesn’t lose anything. Rather, he pauses his own nezirus, starts counting the nezirus period for his son, shaves, brings his offerings, and then completes the 30+ days that remain from his own nezirus, after which he shaves again. If fewer than 30 of the 100 days remain, then he loses some until he has observed 70. (This is explained in the next halacha.)

Hilchos Nezirus 4:5

For example, let’s say that his son was born on day 80. In such a case, he observes the nezirus for his son, finishes it, shaves, then observes another 30 days. We see that he has lost the ten days that were before his son was born, i.e., from day 70 until the birth. The same is true in all comparable scenarios.