How to config timer by number of working days? E.g. trigger a reminder e-mail after 5 working days

In order to customize the timer, you should create a handler for GetCustomTimerValueAsync:

runtime = new WorkflowRuntime();
runtime.GetCustomTimerValueAsync += GetCustomTimerValueAsync;

This handler is called when registering the timer with TimerType = Expression, and the Value string that differs from the format: "2d 5h 24m 15s", "2days 5hours", etc.

Thus, by implementing the handler, you can send and process any values necessary to implement an algorithm of your own.

Example of using the handler:

class TimerValue
    public int WorkingDays { get; set; }

public static async Task<DateTime?> GetCustomTimerValueAsync(string value, string name)
    TimerValue timerValue = null;
        timerValue = JsonConvert.DeserializeObject<TimerValue>(value);

    if (timerValue != null)
        //TODO: Add your algorithm for calculating working days.
        return runtime.RuntimeDateTimeNow.AddDays(timerValue.WorkingDays);

    return null;

In the designer, creation of a new custom timer will look as follows:

Workflow Timer Value