Break dependency on DateTime

In Utilities, add Clock.cs.

public interface IClock
{
    DateTime UtcNow { get; }
    DateTime Now { get; }
}

public class Clock : IClock
{
    private readonly TimeZoneInfo _timeZoneInfo;

    public Clock(IConfiguration configuration)
    {
        var timeZoneName = configuration.Get<AppSettings>().Timezone;
        _timeZoneInfo = TimeZoneInfo.FindSystemTimeZoneById(timeZoneName);
    }
    
    public DateTime UtcNow => DateTime.UtcNow; 
    public DateTime Now => TimeZoneInfo.ConvertTime(DateTime.UtcNow, _timeZoneInfo);
}

Back in BookService, inject IClock and use _clock.Now.Year instead.

public class BookService(ILogger<BookService> logger, IClock clock) : IBookService
{
    public int Create(CreateBookRequest request)
    {
        logger.LogInformation("BookController.Create - Request {@request}", request);
        
        if (request.PublicationYear < 1900 || request.PublicationYear > clock.Now.Year)
        {
            throw new ValidationException("Publication Year must be between 1990 and 2025");
        }
        
        //TODO create book
        var id = 1;

        logger.LogInformation("BookController.Index - Response {@response}", id);
        return id;
    }
}