Updating datetime fields with NULL values

Morning All,

Is it possible to clear a datetime field to NULL? If so, how? In trying an update (update test set d1='' where
t1='Text2';)I get an error of Bad datetime external representation ''
 

TIA,
Gary MacMinn
===============================================
Table    = test
+----------------------------------+----------------------------------+-------+
|              Field               |              Type                | Length|
+----------------------------------+----------------------------------+-------+
| t1                               | text                             |   var |
| d1                               | datetime                         |     8 |
+----------------------------------+----------------------------------+-------+