I use the following script to prevent user to enter an earlier date`function validateField(scriptContext) {`
var curRec = scriptContext.currentRecord;
var fieldName = scriptContext.fieldId;
if (fieldName === 'custrecord_date_souhaitee') { //replace "date" with your field id
var recDate = curRec.getValue({fieldId: fieldName});
var today = new Date();
if (recDate < today){
alert('Vous ne pouvez pas entrer une date antérieure à la date du jour');
curRec.setValue({
fieldId: 'date',
value: null
});
}
}