Hi to everyone, I've read the guide to sail for designer and the vario

Which part do you think is not replicable? You can use the if() expression to display the hidden fields based on the values selected in the start and end date. You can validate against the value on which the start and end dates are stored into. They can either be a rule input or a local variable.
Let me know if this addresses your requirement?

yes the problem is how to use the if function in a sail form to hide or to show some field. I've tried with this:
=a!formLayout(
label: "Untitled",
instructions: "Please fill in the form below",
firstColumnContents: {
if(ri!test, a!textField(label: "Error Message", readOnly: true, value: "The Start Date cannot be before today"), a!textField(label: "Error Message", readOnly: true, disabled: true, value: "The Start Date cannot be befor today")),
},
buttons: a!buttonLayout(
primaryButtons: a!buttonWidgetSubmit(
label: "Submit",
style: "PRIMARY"
)
)
)
where the rule input test is a boolean.
But it doesn't work

That SAIL works fine, though you've got an extra comma after the if() condition that you don't need.