SSIS/SQL - Convert Date/Time to String Failing - sql

Question

I have a basic SSIS job that runs a SQL query and exports the results of the query to an Excel file. The SSIS job has run without issue for over a year, but has started to fail (at validation phase, before execution) this month with the following error:
"Conversion Failed when converting date and/or time from character
string."
The SQL query itself is a simple SELECT with a WHERE clause to specify a start and end date range to gather records from. The part of the query that determines the date range is below:
DECLARE #RunDateTime datetime
DECLARE #RunDate datetime
DECLARE #StartDate datetime
DECLARE #EndDate datetime
DECLARE #Month int
DECLARE #Year int
DECLARE #strStartDate varchar(10)
SET #RunDateTime = GetDate()
SET #RunDate = CAST(#RunDateTime AS DATE)
IF DATEPART(d, #RunDate) = 16
BEGIN
SET #StartDate = DATEADD(d, -15, #RunDate)
SET #EndDate = #RunDate
END
ELSE
BEGIN
IF Month(#RunDate) = 1
SET #Month = 12
ELSE
SET #Month = Month(#RunDate) - 1
IF Month(#RunDate) = 1
SET #Year = Year(#RunDate) - 1
ELSE
SET #Year = Year(#RunDate)
SET #strStartDate = CAST(#Year AS VARCHAR) + CAST(#Month AS VARCHAR) + '16'
SET #StartDate = CONVERT(datetime, #strStartDate)
SET #EndDate = #RunDate
END
I can take this code out of SSIS and execute it directly on the server via SSMS, and it works correctly with no errors. However, the SSIS job cannot run it. In fact, I can't even go into the OLE DB Source object for the SQL command and view the column metadata anymore, I get the error mentioned above every time.
I have found that by commenting out the SET #StartDate = CONVERT(datetime, #strStartDate) line the query works again in SSIS, but there's no reason this line should cause problems. As I said, it has worked for over a year with no issues. I'm literally at my wits' end with this, and cannot understand why the job has suddenly stopped working.
I did check with my DBA, and he assures me nothing has been changed on the SQL server side, but then again I wouldn't even know where to ask him to look.
Other info:
The SSIS job is using the SQLNCLI10.1 connection provider. (2008 R2 SQL Server)
I've tried using other connection providers with no luck.
The date column involved in the query is of type datetime
There have been no regional/localization changes that would affect the dates. (And all the dates are ISO formatted).

Answer 1

This fails because the #strStartDate is not a valid date or is not in a recognized format. It shouldn't fail in November, December or January but it will definitely fail for February through October (Month(#RunDate) - 1) because the CAST(#Month AS VARCHAR) returns only a single digit.
To be honest this is is a rather long way of calculating the 16th of the previous month, can you change it to...
DECLARE #StartDate DateTime
IF DATEPART(d, GetDate()) = 16
SET #StartDate = DateAdd(mm, DateDiff(mm,0,GetDate()),0)
ELSE
SET #StartDate = DateAdd(mm, DateDiff(mm,0,GetDate())-1,0)+15;
So much easier