LocalizationProvider Class

Implement ILocalizationProvider:

private sealed class UpperCaseLocalizationProvider : ILocalizationProvider
{
    public string GetString(string name, CultureInfo? culture)
    {
        return ResxLocalizationProvider.Instance.GetString(name, culture).ToUpperInvariant();
    }
}

Use it for all RelativeDate instances:

// Use custom localization provider
LocalizationProvider.Current = new UpperCaseLocalizationProvider();

var date = RelativeDate.Get(DateTime.UtcNow.AddHours(-2));
Assert.Equal("2 HOURS AGO", date.ToString(null, CultureInfo.InvariantCulture));

// Restore default
LocalizationProvider.Current = ResxLocalizationProvider.Instance;