Datetime in primary key index

I am migrating an oracle Database to mysql.

I have an composite unique index on a varchar and a datetime.

for some reason though when I do the loading of data mysql only recognises the first 15 characters of the datetime field.

e.g.

for the fields
custnum 101000
date 2001-01-19 10:15:26

it will check for uniqueness on

101000-2001-01-19 10:10:00

so when I have another date like 2001-01-19 10:17:12

I get a non unique error

any ideas?

thanks in advance

Will