However, if they are asking for a W9, they are going to treat you as an Independent Contractor and not an Employee.
If you work the hours that they state, using their tools, equipment and supplies, and under their supervision, you should probably be an Employee.
No you need one signed this year. And if you aren't a 1099 contractor your shouldn't be sending a W9.
Your employer? Are you an employee? If so, you send a W-4. If you are being treated like an employee in the work place you send a W-4. If you are being treated ;like an employee in the workplace and your employer insists on a W-9 contact your state labor department and unemployment tax department and let them sort it out.
If they are your employer, then you must use Form W-2. If they are your employer, then Form W-9 is not allowed.
If you are an independent contractor (not an employee), then Form W-9 is correct, but you should not call them "an employer", because they are not your employer.
No, that would be FRAUD….
Here's a W9: https://www.irs.gov/pub/irs-pdf/fw9.pdf
Fill it in and send it to them (like you SHOULD have done before doing any WORK for them!)
yes, you can sign it and date it, that is the effective date