Please note that to be valid, you have to enclose the value within single quotes. For example, say you have the following (totally useless) schema:

Foobar:
  columns:
    published_at: { type: timestamp }

And you want to create fixtures for this table. You might go this way (please also note that symfony allows php in fixtures files):

Foobar:
  foobar_1:
    published_at: <?php echo time(); ?>

Which won't work, since the Doctrine_Validator_Timestamp expects a date in the Y-m-d H:i:s format. So maybe you'll try this one:

    published_at: <?php echo date('Y-m-d H:i:s'); ?>

Which still doesn't work, since the value gets converted to an unix timestamp.

The right way is (note the enclosing single quotes):

   published_at: '<?php echo date('Y-m-d H:i:s'); ?>'

Yay o//

This behavior is explained in this ticket might help.