Fix issue with lead time not being added

[krozendaal]

See this issue for more information: tastyigniter/TastyIgniter#1004

[krozendaal]

Do not merge this just yet, please.
I just discovered a major flaw: once the end time is being reached, this code throws an error. I will try to fix this issue today, and update my code.

[krozendaal]

I think a seperate extension suited better here, as multiple methods need rewriting to get this done, whereas a simple extension fixes the issue. On top of that, this is a much safer option for me instead of changing core files. I created the extension here: https://github.com/krozendaal/ti-ext-fixleadtime which is free to use for everyone.