datetime - Extract only date from gridview -

datetime date = datetime.parseexact(datagridview1.rows[i].cells[0].value.tostring(), "yyyy-mm-dd", system.globalization.cultureinfo.invariantculture); 

how extract date code?

while executing showing this

an unhandled exception of type 'system.formatexception' occurred in mscorlib.dll  additional information: string not recognized valid datetime.