How always is there someone that makes katas in one line?

You don't need the 'else' statements. If your if statements have 'return' in their conditions (like this case) and there's only one statement that can be true (the person cannot have multiple ages so it doesn't pass through multiple if statements), you don't need the 'else' keyword. You can simply write 'if'.

It's naïve to think there are only 26 different letters, even using english.

Why do you apply ToLower on each character? String has that method too. Meaning you could've started with str.ToLower() and omitted that Select call.

Yep, in the tree that is described even in the example, this solution returns the incorrect value "15" instead of "-51".

Probably requirements did not before contain data about the possible values of h, m, s. Now they are.

"oh yeah duh" - me

I'm glad I had the exact same solution :)
Notice that you don't need the second paranthesis around the timespan instantiation

This is clean and simple.

One of the important Perks of Reading Documentation before Concept Imlpementation, A lot of time save :D

nice

Something ain't right when 2 comes before n haha.