How to compare approximate dates with SQL

Let say you have to write a code for the periodic payment application, and in your database you store the payment start date. After that you want to check if the next payment is within +/- 1 day of the current date - do something. And obviously, you font want to write a lot of code to do that :)

The first idea I had was to create a whole bunch of unions for each day and then check if current date is one of those, but later I shrinked that to 3 conditions connected with 'or' (here is NHibernate function for that)

var settlementDateDifferenceFromNow = Projections.SqlFunction(
 new SQLFunctionTemplate(
  NHibernateUtil.Int32,
  @"
  CASE WHEN
  ABS(DATEDIFF(DAY, GETDATE(), DATEADD(MONTH, DATEDIFF(MONTH, ?1, GETDATE()) - 1, ?1))) <= 1 OR
  ABS(DATEDIFF(DAY, GETDATE(), DATEADD(MONTH, DATEDIFF(MONTH, ?1, GETDATE()), ?1))) <= 1 OR
  ABS(DATEDIFF(DAY, GETDATE(), DATEADD(MONTH, DATEDIFF(MONTH, ?1, GETDATE()) + 1, ?1))) <= 1
  THEN 1
  ELSE 0
  END
  "),
 NHibernateUtil.Int32,
 Projections.Property<Contract>(x => x.SettlementDate_Actual)
);

I am not sure about 'or' conjunction tho... would it cause performance problems?