Powershell $cmd.ExecuteNonQuery() changes DATETIME to NVARCHAR - sql

Question

Caveat: I am new to PowerShell...
I am trying to automate a load process for 48 tables. The table names are all of the format "Vocabulary_MMddyyyy", as in "Vocabulary_12052016". I have no trouble iterating through the table names and parsing out the MMddyyyy part (the release version for the vocabulary). Powershell does not recognize the '12052016' as a proper datetime format. So, I do some parsing to build a proper date string that I convert to DateTime with the Get-Date cmdlet:
$version = ($table.Table_name -replace 'Vocabulary_', '').Trim();
$mm = $version.Substring(0,2);
$dd = $version.Substring(2,2);
$yyyy = $version.Substring(4,4);
$date = $mm + "/" + $dd +"/" + $yyyy
$startDate = Get-Date -Date $date -Format "d"
$endDate = ($startDate).AddDays(-1)
write-host 'Version: ' $version
write-host 'StartDate: ' $startDate
write-host 'EndDate: ' $endDate
This properly sets $startDate and $endDates (though despite the "d" format, I still get HH:MM:SS AM part):
Version: 12052016
StartDate: 12/5/2016 12:00:00 AM
EndDate: 12/4/2016 12:00:00 AM
This is the SQL signature of the stored procedure I want to call:
CREATE PROCEDURE [vocab].[prInsertLoadProcessID] (
#CodeQualifierID INT,
#SourceVersion VARCHAR(100),
#SourceDate DATE,
#LoadTypeID INT,
#isTestLoad BIT,
#ValidationDate DATE,
#LoadDescription VARCHAR(500),
#GlobalEffectiveDateStart DATE,
#GlobalEffectiveDateEnd DATE
)
Now, I go to build up a SQL command to execute the stored procedure ($conn2 is properly defined and open; it executes simple select statements yielding correct results):
$cmd.Connection = $conn2;
$cmd.CommandText = "vocab.prInsertLoadProcessID ";
$cmd.CommandType = [System.Data.CommandType]::StoredProcedure;
$cmd = New-Object System.Data.SqlClient.SqlCommand
$cmd.Connection = $conn2;
$cmd.CommandText = "vocab.prInsertLoadProcessID ";
$cmd.CommandType = [System.Data.CommandType]::StoredProcedure;
$cmd.Parameters.AddWithValue("#CodeQualifierID", 6);
$cmd.Parameters.AddWithValue("#SourceVersion", $version);
$cmd.Parameters.AddWithValue("#SourceDate", $startDate);
$cmd.Parameters.AddWithValue("#LoadTypeID", 1);
$cmd.Parameters.AddWithValue("#isTestLoad", 0);
$cmd.Parameters.AddWithValue("#ValidationDate", 'NULL');
$cmd.Parameters.AddWithValue("#LoadDescription", $LoadDescription);
$cmd.Parameters.AddWithValue("#GlobalEffectiveDateStart", $startDate);
$cmd.Parameters.AddWithValue("#GlobalEffectiveDateEnd", $endDate);
Each of the dates (#SourceDate, #GlobalEffectiveDateStart, #GlobalEffectiveDateEnd) show as DateTime when I look at the output from adding the parameters (here is one example):
CompareInfo : None
XmlSchemaCollectionDatabase :
XmlSchemaCollectionOwningSchema :
XmlSchemaCollectionName :
ForceColumnEncryption : False
**DbType : DateTime**
LocaleId : 0
**ParameterName : #GlobalEffectiveDateEnd**
Precision : 0
Scale : 0
**SqlDbType : DateTime**
SqlValue : 12/6/2015 12:00:00 AM
UdtTypeName :
TypeName :
Value : 12/6/2015 12:00:00 AM
Direction : Input
IsNullable : False
Offset : 0
Size : 0
SourceColumn :
SourceColumnNullMapping : False
SourceVersion : Current
When I run $cmd.ExecuteNonQuery() I get:
Exception calling "ExecuteNonQuery" with "0" argument(s): "Error converting data
type nvarchar to date." this is a Category: MethodInvocationException and a FullyQualifiedErroID: SQLException.
So, after reading for hours and trying everything I can think of I am stuck. My variables appear to be typed as DATETIME, but ExecuteNonQuery() seems to read treat them as NVARCHAR.
What am I missing? I can certainly rewrite the stored procedure to take strings instead of dates and let SQL do the conversion, but it really seems I should be able to pass dates from Powershell to SQL.
There must be a solution to this. What is causing the $cmd.ExecuteNonQuery() to change the datatype of my inputs? Or, am I setting up the $cmd.Parameters incorrectly?
Kind thanks for any help.