How do oracle date time work?
BETWEEN is inclusive. From the documentation:


  And the value of

expr1 BETWEEN expr2 AND expr3

  
  is the value of the boolean expression:

expr2 &lt;= expr1 AND expr1 &lt;= expr3



So your first clause will match exactly T1, exactly T2, and anything in between. Your second clause will match exactly T1, i.e. any rows where DATETIME is exactly T1.