ITrackable.cs

public interface ITrackable
{
    DateTime CreatedAt { get; set; }
    DateTime UpdatedAt { get; set; }
}

TimestampMethods.cs

public override int SaveChanges(bool acceptAllChangesOnSuccess)
{
    SetTimestamps();
    return base.SaveChanges(acceptAllChangesOnSuccess);
}

public override Task<int> SaveChangesAsync(bool acceptAllChangesOnSuccess,
    CancellationToken cancellationToken = default(CancellationToken))
{
    SetTimestamps();
    return base.SaveChangesAsync(acceptAllChangesOnSuccess, cancellationToken);
}

private void SetTimestamps()
{
    IEnumerable<EntityEntry> entries =
        ChangeTracker.Entries()
            .Where(entry => entry.Entity is ITrackable
                            && (entry.State == EntityState.Added
                                || entry.State == EntityState.Modified));

    foreach (EntityEntry entry in entries)
    {
        var model = (ITrackable)entry.Entity;
        DateTime now = DateTime.UtcNow;

        if (entry.State == EntityState.Added)
            model.CreatedAt = now;

        model.UpdatedAt = now;
    }

    ChangeTracker.Entries()
        .Where(entry => entry.Entity is ITrackable
                        && (entry.State == EntityState.Added
                            || entry.State == EntityState.Modified));

    foreach (EntityEntry entry in entries)
    {
        var model = (ITrackable)entry.Entity;
        DateTime now = DateTime.UtcNow;

        if (entry.State == EntityState.Added)
            model.CreatedAt = now;

        model.UpdatedAt = now;
    }
}