With OpenTTD Timetable Tools app you can easily calculate timetable for any number of vehicles. You need to enter timetable start date, vehicle timetable length and select if you want to enter frequency or number of vehicles. Program will count if timetable length needs to be increased, how many vehicles will be needed and their departure date. You can select Date format in Settings.
This is new Pc version of app. It is doing exactly same things like Android version. There are only minor differences, most obviously different look. App is runnable JAR file and requires JRE 1.6 (or higher) installed. Application was tested on Windows 7 and Linux Mint.
You can download app here.