Query regarding current_time and checksum... plz help

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Query regarding current_time and checksum... plz help

Hi,

I am very new to teradata. I am using mainframe+teradata and also thru sql assistant. Today I observed something for which I want an explanation. two problems--
Prob 1. I had a table tablea, where a field operational_tim was declared as integer. But I was supposed to change this column to the below format.
OPERATIONAL_TIM TIME(6) NOT NULL DEFAULT CURRENT_TIME(0).
The table has data. So, I did below things
ALTER TABLEA
DROP OPERATIONAL_TIM
ALTER TABLEA
ADD OPERATIONAL_TIM TIME(6) NOT NULL DEFAULT CURRENT_TIME(0)

First alter ran fine but the second one gave me error as:
incompatible default value. Plz help me regarding this

Prob 2. I created one table TableB with one column as
OPERATIONAL_TIM TIME(6) NOT NULL DEFAULT CURRENT_TIME(0)
got the same error as incompatible default value and then with a trial and error,when defined the table with CHECKSUM=DEFAULT option the table got created succesfully.
Can anybody explain why with checksum=default option it got created successfully?