Ich werde noch heute den Arzt anrufen.
Noch im Mai hat sie ihre Dissertation abgegeben.
These are from Hammer's German Grammar and Usage 4th Edition p203, where it is noted that this way of using 'noch' differs significantly from the other cases where there is an undertone similar to the english 'still', and that 'noch' here instead puts particular emphasis on the fact that the speaker will call the doctor e.g. today, rather than tomorrow.
I would like to ask, would anyone have further explanations about what exactly 'noch' used in this sort of sense means? e.g., what context or additional meaning would be suggested by the fact that we're using 'noch' in these examples, instead of leaving it out? Unfortunately, the commentary in Han's Grammar here is particularly short.