QotD

Today's Question of the Day...

Assume a) hot coffee and b) cold milk.
Further assume you want to mix both ingredients, and keep the mixture maximally warm for as long as possible. Specifically: After being supplied with both ingredients, there is a time span t_d after which the mixture should be maximally warm.

The question is:
When should mixing occur - early on, i.e. at t = 0, or late, i.e. near t = t_d?

(Actually, I posed the question to different people several times already. Unfortunately, I keep forgetting the answer.
For results, please mail them in to philipp@stolka.de.)