date_in function

I want to change the date from a field in a tuple in a trigger_function
create table example (    my_date datetime    ...
);

int na;
char select[20];

na = SPI_fnumber(trigdata->tg_relation->rd_att, "my_date");
memset(select, 0, sizeof(select));
sprintf(select, "1/1/2002");
newval = DirectFunctionCall1(date_in, CStringGetDatum(select));
rettuple = SPI_modifytuple(trigdata->tg_relation, rettuple, 1, &na, &newval, 
NULL);
if(rettuple == NULL) elog(ERROR, "my_function: %d returned by
SPI_modifytuple", SPI_result);

when i test that my server goes down. What I missed ?

Not my function is about, but date_in.
Have anybody worked with this function ?
Can somebody give me a sample to use date_in. Thanks.