Date and Time Help

I've got a file of around 12,000+ of which one column is the year. from "0. AD" to current "2021.AD". However, I have two problems I have encountered.

1. Presently I just have the year format. However I need to input in through a CSV file into QGIS 3.8 using TimeManager and need it in one of the following formats, I know I can just use the formats settings however this changed the date randomly to the example shown. I need it in this format "YYYY-MM-DD"

2. I believe the year I need a four figure date e.g. for the year "4.AD" I should have entered "0004" and not just "4" like I have done, is there a way to resolve this rather than going through one by one and changing them?

Any help appreciated

Sorry to sound silly aha, is this response for 1. or 2? Which parts do I need to amend for my sheet/columns

Thanks