how to disable save entry on alert?

how to disable save entry on alert?

hi!

just made alert On User Input
example:

if (input.datadostavki  >=  zoho.currentdate.addDay(3))
{
    alert("alert!");
}

how can i disable save entry when there is true "if (input.datadostavki  >=  zoho.currentdate.addDay(3))" ?
in this time user can see alert, close it and save changed entry

i need when there is true "if (input.datadostavki  >=  zoho.currentdate.addDay(3))" - disable to user to save entry.

please help to make this.