SQL: Error using DATEFROMPARTS to add day from another date variable - sql

Question

everyone:
I am trying to compare two dates, but they are given in two different formats:
[Open Date] given YYYY-MM-DD
[Monthly Fee Year] given in YYYY as int
[Monthly Fee Month] given in MM as int
I need to add the Day from Open Date to Monthly Fee. I just found out about DATEFROMPARTS, so I tried this:
SELECT [OPEN DATE]
,[Monthly Fee Year]
,[Monthly Fee Month]
,DATEFROMPARTS([Monthly Fee Year], [Monthly Fee Month], DAY([OPEN DATE]))
FROM [#temptable]
I know that the parameters for DATEFROMPARTS should all be int, and I believe all the parameters I put in are int, but I got this error even after I use CAST():
"Cannot construct data type date, some of the arguments have values which are not valid."
Here is some sample data:
OPEN DATE Monthly Fee Year Monthly Fee Month
2021-06-08 2021 8
2019-12-17 2020 3
2019-05-30 2020 3
2020-10-08 2021 2
2021-05-28 2022 3
And I want to add a column with:
Monthly Fee Date
2021-08-08
2020-03-17
2020-03-30
2021-02-08
2022-03-28
UPDATE: It turns out that Monthly Fee Year and Monthly Fee Month are nvarchar, so I tried to CAST or CONVERT them as int:
,DATEFROMPARTS(CAST([Monthly Fee Year] AS int), (CAST([Monthly Fee Month] AS int), DAY([OPEN DATE]))
,DATEFROMPARTS(CONVERT(int, [Monthly Fee Year]), CONVERT(int, [Monthly Fee Month]), DAY([OPEN DATE])
But I still get the same error.
What am I missing?
Thank you.

Answer 1

Your sample data is fine, which is throwing us off. DATEFROMPARTS() will also implicitly convert your NVARCHAR types to INT, so that's not your issue. Your problem is likely that you are trying to create invalid dates. For example, if the Open Date is 2021-05-31 and the Monthly Fee Year is 2021 and the Monthly Fee Month is 9, you are trying to create a date of '2021-09-31', which isn't valid.
You'll need to come up with some logic or a rule to deal with dates like that.
Edit:
If you get the first of the month for each date you are trying to build and find out how many months away from the Open Date that is, you can add that many months to Open Date and get what you are trying to accomplish, but the invalid dates would be handled properly.
, DATEADD(MONTH, DATEDIFF(MONTH, [Open Date], DATEFROMPARTS([Monthly Fee Year], [Monthly Fee Month], 1)), [Open Date])

Answer 2

This should work, not very elegant but will stop it from trying to create invalid dates:
IIF(DAY(EOMONTH(DATEFROMPARTS([Monthly Fee Year], [Monthly Fee Month], 1))) < DAY([OPEN DATE]),DATEFROMPARTS([Monthly Fee Year], [Monthly Fee Month],DAY(EOMONTH(DATEFROMPARTS([Monthly Fee Year], [Monthly Fee Month],1)))),DATEFROMPARTS([Monthly Fee Year], [Monthly Fee Month], DAY([OPEN DATE])))
First check if the last day in the month is less than the day of the open date
If it is then use the last day in month as the day
If not then use the day of the Open Date