Compiler throws "ambiguous overload for operator"

I'm learning how to use std::chrono and I want to make a template class Timer easy to use (defined in timer.h). The testing program was successful and everything worked fine, until I tried to use my new Timer in a program with the definition of some template operators, which conflit with the operators used inside Timer.

Inside Timer I have to use operator- between two variables (start_time and end_time) of type std::chrono::time_point, in order to obtain the duration variable containing the elapsed time.

In another header (algebra.h) I implemented the overloading of the binary operator- to make the difference between two std::vector or two std::array, or also a user-defined container provided with operator[] and size() member function.

template<typename pointType>
pointType operator-(pointType a, const pointType & b){
    for(int i = 0; i < a.size(); ++i){
        a[i] = a[i] - b[i];
    }
    return a;
}

When I try to include both timer.h and algebra.h, the compiler throws an error saying "ambiguous overload for operator-" suggesting, as possible candidates, both the operator in algebra.h and the one implemented in <chrono>.

I don't understand why it is ambiguous, since pointType can't be deduced as std::chrono::time_point because it doesn't have operator[] and size() member function.

P.S. I tried something else to work it out, but I only got more confused testing a program which use std::valarray. When I include both <valarray> and "algebra.h", and try to make a difference between two valarrays, I expected the compiler to complain about ambiguous definition of operator-, since std::valarray already has implementation for binary operators. But this doesn't happen: it compiles using the <valarray> implementation. Why this doesn't throw an error?