l have a MySQL table where I created the date column as datatype DECIMAL(11,6). I realise now that this was a silly thing to do.

The date looks like this.
40467.778796 where the integer part is the date and the decimal part is the part of the day.
The number 40467.778796 is equivalent to 16/10/2010 18:41:28

The table stores realtime data from a turbine. A new row is added every second, with the id, date as a decimal as described, and 150 datapoints.

It is very slow to search for records based on the decimal date that I have used.

I would like to create a new DATETIME column in the database and set this as a primary key so I can search quickly on it. Is this the right way to go about this?

I wish to copy all the existing decimal dates into this new DATETIME column.

How would I go about doing this? I understand how to create a new column and set it as the primary key, but I need some help to understand:
1. How do I convert the figures in my decimal column to a DATETIME format?
2. How do I do this for all existing records in the table?

Thank you