writing functions in angularjs services

Expose the function as a service, then let the AngularJS injector do the rest. You can easily set a daysInMonth service as a static value in your module. See this in action at http://jsfiddle.net/hbulhoes/zdtnw/

var mod = angular.module('myapp', []);

// This is the declaration of the daysInMonth service. It's set as
// a value in the module, with the value being the very function
// you want to share among other services and controllers:
mod.value('daysInMonth', function(month, year) {
    return new Date(year, month+1,0).getDate();
});

// And this is an example controller that depends on the daysInMonth function.
function MyController($scope, daysInMonth){
    $scope.DaysInCurrentMonth = daysInMonth(12, 2012);
}

Here is fiddle with a basic example of how you can use (inject) services in controllers.

http://jsfiddle.net/uhmNR/1/

var myApp = angular.module('myApp',[]);


//Here is the service Users with its functions and attributes
//You can inject it in any controller, service is a singleton and its data persist between controllers
myApp.factory('Users', function () {

    var userName = "John Doe";

    return {
        getUserName: function () {
             return userName;                   
        },
        setUserName: function (newName) {
            userName = newName;
        }
    }
});

//An Util service with DaysInMonth method   
myApp.factory('Util', function () {

    return {
        daysInMonth: function (month,year) {

            return new Date(year, month+1,0).getDate();
        }
    };

});   

//Here I am injecting the User service andusing its methods   
myApp.controller('MyCtrl', ['$scope', 'Users', 'Util', function ($scope, Users, Util) {

    Users.setUserName('Robin Hood');

    $scope.name = Users.getUserName();

    //Using Util.daysInMonth()
    $scope.date = Util.daysInMonth(12,2012);
}]);

Hope It helps.