It's time = A simple statement that it is time for something. It's about time = Often said angrily as a message of impatience; for example, if someone comes late you say "It's about time you're here!" (More technically, if said without anger/frustration, it also can mean it is around the time for some event, even if the time is not known exactly) It's high time = I have never heard that expression, so it must be native to other English speaking areas.

As I'm sure you've learnt, all these expressions take a subjunctive in the form of a 'past tense' verb. All of these are talking about present time, but the subjunctive form indicates an 'unreal present'. Maybe we're not really going now, but it's certainly time we went. I'd agree with Samuel as to the differences between them: 1. It's time we went. This is a neutral statement 2. It's about time you found yourself a job. This is often said in angry or impatient way, or by someone jokily pretending to be angry or impatient. Imagine a mother talking to a lazy grown-up son. 3. It's high time I bought a decent diary. This is your own example. And once again, I'd agree entirely with Samuel - the form is indeed used in other English-speaking areas. It's certainly used in British English, although it is a little old-fashioned. And yes, the 'high' does add emphasis. The implication here is that it's more than time - or even a little late. If you say ' It's high time I bought a decent diary.', you're suggesting that your diary is completely useless and falling apart - and probably has been for quite a long time! I hope this helps.