In my Parsha Challenge for Parshas Massei, I pointed out that according to Rashi who advocates a one-date-fits-all (RH) method for calculating ages for the national census, it is impossible yto say that this applies to when the Torah reports the ages of Moshe and Aharon in Shmos and BaMidbar. We prefer not to say that the Torah is needlessly inconsistent. So, either we find an explanation for the inconsistency, or we have an issue with Rashi. This would be in addition to Ramban's issue with Rashi that the Gemara in Arachin 18b explicitly says that the age definition for Arachin is the precise date.
As I left off, I had no explanation for Rashi or the inconsistency. Fortunately, I am currently learning Arachin, and with a brilliant chavrusa to boot (Rabbi Yehoshua B.). He wanted to suggest that perhaps the Torah bases age on the precise date when dealing with an individual. But, when dealing with a tzibbur, such as the national census, only there does the Torah work with a single collective "birth date". For Arachin, wherein one points to an individual and vows his value to hekdesh, we calculate him as an individual.
This would alleviate Ramban's challenge to Rashi and give an explanation for the Torah using different systems. It does not fully answer our problem with Moshe's age at Shmos 7:7 assuming that the audience was before 7 Adar 2448. For this we must still accept that the Torah is willing to Fall back on the Miktzaso K'Kulo method. Perhaps, we can say that since no Halacha is involved in Shmos 7:7, the Torah is more flexible with age calculations.
I hope Rashi is smiling.