Found the answer in the KDE Help Center: %{date}
is a Kate template macro (Kate is the editor used by KDevelop). Its expansion depends on the system locale, and there is no indication about a way to change the locale. By using other macros, however, I can obtain the desired result:
%{year}-%{month}-%{day}
For more complex cases, I suspect I'd need some more digging into the Kate Template Format, but the information so far satisfies my immediate need.