If you’re trying to create column validation using [Today], you’ll likely get the following error:
Sorry, something went wrong. The formula cannot refer to another column. Check the formula for spelling mistakes or update the formula to reference only this column.
Commit this to memory: when [Today] or Today fails in a formula, try Today().
While [Today] may work in list view filters, it’s not the correct format for column validation or calculated columns. You can, instead, use Today() for column validation.
For example, you could use EndDate<=Today() to require users to enter an EndDate that is on or before the date of submission.
Note that if you’re creating column validation through the modern UI, you’ll need to start validation formulas with an equals sign.