Also you may need to update the formula for your date calculation. I don't remember if ADD_MONTHS will just change the month to the next one, or if it will attempt to validate the new month comparison. (I'm not sure if adding 1 month to 3/31/2024, will try to set the date as 4/31/2024, which does not exist).
If you get an error you can also try this LAST_DAY({shipdate} + 1)....essentially just add any number to the ship date to get it to roll into the next month (so 1 or 5 or 10 or something), then use the LAST_DAY funciton to get that last day of that new month. This should fix any errors and give you the last day of the next month