I don't know why people have voted down @nurdin's answer - he's absolutely right. Of course, 'today' does mean the period of 24 hours that we're in at the moment. Obviously, it's correct to say 'It's October 3rd today'. This is basic English and I'm sure that SJH realises this! However, 'today' can also refer to present times, in the sense of the current period of time or even the modern era. For example, 'In the past most people lived in the countryside and in small villages. Today the majority of the population lives in cities.' When used in this sense, 'today', 'nowadays', and 'these days' have exactly the same meaning. SJH seems to have an advanced level of English, so it's likely that this is what her question is about. I've just given @nurdin an upvote.

In my mind. All of them are same means thats presen times,so it depends on condition wich one do you choose.

Hello SJH! I'll say some sentences: -"My bike was damaged by an accident yesterday, so I have to go school by bus these days". As you can see, "these days" means some days in present. -"My province was a countryside 10 years ago. Nowadays, It is a crowded city". As you can see, "nowadays" means present. -"I usually go to school by car. But, I went to school by bus today". As you can see, "today" means one day in present.