If you don't want to use the KnockoutValidation library you can write your own. Here's an example for a Mandatory field.
Add a javascript class with all you KO extensions or extenders, and add the following:
ko.extenders.required = function (target, overrideMessage) {
//add some sub-observables to our observable
target.hasError = ko.observable();
target.validationMessage = ko.observable();
//define a function to do validation
function validate(newValue) {
target.hasError(newValue ? false : true);
target.validationMessage(newValue ? "" : overrideMessage || "This field is required");
}
//initial validation
validate(target());
//validate whenever the value changes
target.subscribe(validate);
//return the original observable
return target;
};
Then in your viewModel extend you observable by:
self.dateOfPayment: ko.observable().extend({ required: "" }),
There are a number of examples online for this style of validation.