Can be Purchase Order is post invoice date

Hey there, gramaswamy.

I'm glad you turned to the QuickBooks Community for help about adding purchase order date to an invoice. 

Are you referring to adding a purchase order date custom field on the invoice? If so, QuickBooks Online (QBO) allows you to customize a sales transaction through your company settings.

Let me show you how:

1. Click on the Gear icon.
2. Select Account and Settings.
3. Go to Sales. 
4. In the Sales form content section, click on the pencil icon to edit and go to Custom field section. 
5. Then enter the name of the custom field. 
6. Check the Internal box if you want to show the field in QBO. Choose Public if you want to show the field on customer forms.
7. Click on Save and then Done.

To learn more about adding custom fields to invoice, you can refer to this article: Add custom fields in QuickBooks Online (QBO).

There you have it. You should be able to add custom fields, such as purchase order date, on the invoice. 

Feel free to click on Add comment below with more details if you're referring to something else. I'll be here to help.