question about file write time update?

[ssdl.566]

 Hi  Jeremy:
              First of all, thank you very much for your replies to my previous questions.
  

              Recently, I have a question about write time update on normal files.
              I noticed that the file will be marked modified 2 second later after first write , and the write time didnot get changed for subsequent writes until close.
              I want to know why? is this the requirement by protocol? for what reason?
             another question: 
              is it possible that samba donot manage write time in tdb, just store and fetch file write time  from underlying filesystem instead? 


Peace and happiness
Leilei dong