Modules & VBA :: SQL Server - Conversion Failed When Converting Date And / Or Time From Character String

I have two table

1. dbo.period (OpeningDate, ClosingDate)
2. dbo.data (blah blah, doc_date)

I want to create a view as follows

Select doc_date from dbo.data
where doc_date> 'select OpeningDate from dbo.period'

both doc_date and opening date have the same format

but the error will still appear as follows:
"Conversion failed when converting date and / or time from character string."