erpnext/docs/dev/custom_script_examples/docs.dev.custom_script.validate.md

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Date Validation: Do not allow past dates in a date field

cur_frm.cscript.custom_validate = function(doc) {
    if (doc.from_date < get_today()) {
        msgprint("You can not select past date in From Date");
        validated = false;
    }
}