[SQL] Handling NULL Dates

Hello All,

I'm trying to find out if PostgreSQL has a function like NVL that replaces null values with a default?

I have a table with datetime fields, some of which are null. I'm trying to extract the date and time components with
thedate() and time() functions but the null values break these functions.
 

If NVL is not there, could anyone suggest alternatives?

TIA,
Gary MacMinn

================
Table    = test1
+----------------------------------+----------------------------------+-------+
|              Field               |              Type                | Length|
+----------------------------------+----------------------------------+-------+
| t1                               | text                             |   var |
| d1                               | datetime                         |     8 |
+----------------------------------+----------------------------------+-------+

Query : SELECT t1, date(d1) from test1;  //The date() function breaks if the d1 field is not set.