Yes, you can estimate the date if you have a general idea but no definite documents to provide the exact date.
And zero might be a good call since you don't know the cost basis that was included in your W-2, however there are many websites that can give the value of stock on a given day to give you an idea. It's up to you, as long as you use reasonable data it's not going to be an issue to have some cost basis even if you use the lowest by checking a few different dates. Then keep the information with your tax file.
@Rod221
[Edited: 03/07/2022 | 1:17p PST]
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