I think you make an interesting point here, but then your example is exactly backwards.
If you have a timestamp defined as days since January 1, 1970: If you do basic arithmetic to answer the question "How many days has it been since Nixon resigned" you will _always get the right number_. There are no leap days, they are just normal days.
The problem only comes in when you try to convert between this date type and other types. Our "days since the epoch" date type is fully internally consistent. As long as you know the correct value for "the day Nixon resigned" and "now", it's just a subtraction.
If you have a timestamp defined as days since January 1, 1970: If you do basic arithmetic to answer the question "How many days has it been since Nixon resigned" you will _always get the right number_. There are no leap days, they are just normal days.
The problem only comes in when you try to convert between this date type and other types. Our "days since the epoch" date type is fully internally consistent. As long as you know the correct value for "the day Nixon resigned" and "now", it's just a subtraction.