Implementing a Yearmonth class
I only have a few small remarks to make:


In your header file, don't include &lt;iostream&gt;: include &lt;iosfwd&gt; instead which contains the forward declarations for every type in &lt;iostream&gt;.
Also, you only use std::ostream in your source file, so you could simply include &lt;ostream&gt; there.
Several times, you compute mymonth / 12 and mymonth % 12. If your class is designed to be used intensively, you could consider using std::div(mymonth, 12) which will compute both values at once and may therefore be slightly faster (if you really need it).
You may want to prefix time_t, time, tm and localtime with std::. Them coming from the C standard library doesn't prevent you to use std::.
Using a const unsigned parameter seems pretty useless. I don't have strong opinions on the const on value parameters, but you could safely drop it since it adds little value.