Hola Barbara

Even though saying "es genial que" (it's great that) might seem to express a fact or truth to the speaker (i.e., their positive evaluation of the situation), it's actually expressing a subjective opinion or attitude towards a hypothetical or desired state of affairs, not a certain, factual state. The subjunctive mood is used to reflect this layer of subjectivity and potential non-reality. Essentially, it's about how the speaker feels about a situation or wishes it to be, rather than stating an objective fact about it.

Even though it might be a fact that "you have free time", the speaker is expressing their positive evaluation of this fact — their subjective opinion. The subjunctive "tengas" is used because the statement is about the speaker's attitude towards the situation rather than the situation itself.

I hope you find this answer useful.

Saludos

Silvia