>Is there a performance difference in creating an index on a date field as such?
>
>1) index on date tag date
>2) index on dtoc(date) tag date
>3) index on dtos(date tag date

Yep. Number 1 is the way to go unless the index is compound and the other field is a string. This is because the others require a function to be evaluated each time the index is re-built or a record is added.

Number two should be avoided since you probably won't get the records in the order that's desired. Unless the optional 1 parameter is added, the result of the function will be MM/DD/YYYY (unless SET DATE is set to something different than AMERICAN or MDY).

Number three will return a string in the format of YYYYDDMM (the same as DTOC(date, 1)), but should be only be used when the index expression contains multiple string fields.