Start Task at specific time

I have an class which derives from IHostedService. In here I have the override method StartAsync as shown below:

public Task StartAsync(CancellationToken cancellationToken)
{
    _timer = new Timer(DoSomething, null, offset, TimeSpan.FromHours(1));
    return Task.CompletedTask;
}

In here I have the variable offset which is of the TimeSpan class.

What I need:

I need to calculate the offset in such way that the task "DoSomething" will be executed every start of an hour, like: 10:00, 13:00, 21:00 or 00:00. So this offset needs to be in minutes.

Example :
Let’s say the application starts running at 13:48PM, the correct offset would be 12 minutes and the next hour would be 14.

My question:
How would I calculate the offset which is in minutes until next hour ?

>Solution :

something like

var minutes = (DateTime.Today.AddHours(DateTime.Now.Hour+1)-DateTime.Now).TotalMinutes

should do the trick