Gap or overlap is not allowed for standard address.
It is a standard behavior of CRM system.
And this is the reason you get errors like below.
- Address is standard address; restriction to validity not allowed
- Validity period incompatible with validity of address
- Restricting the validity of a standard address is not allowed
I'll do some test to help you to understand it.
Case 1
I'll change "Valid From" of a standard address from following screen.

I changed it from "01.01.0001" to "01.01.2015". It can be changed without any problem.
Case 2
This time, I'll change the the "Valid From" of a standard address from following screen.
What would be the result ?

4. Following error occurs.

What is the difference between two scenarios ?
In case 1, If we change the “Valid From” to “01.01.2015”, the duration “01.01.0001 – 31.12.2014” of a standard address becomes empty.
But as we know, time gap is not allowed for standard address.
So system tries to find another non-standard address to fill the duration.
As a result, address “Sendo3” is separated to 2 parts.
One is a standard address with duration “01.01.0001 – 31.12.2014”, the other one have duration “01.01.2015 – 31.12.9999”.
But in case 2, when system tries to find another non-standard address to fill the duration.
System can determine two addresses -- sendo2 and sendo3.
As a result, error occurs. Because system don't know which address (sendo2 or sendo3) should be separated.
Dean Yan