timegetter.cs

public class CurrentTimeGetter
{
    private volatile Holder _holder;
    const String rfc1123Pattern   = "ddd, dd MMM yyyy HH':'mm':'ss 'GMT'";

    public string GetCurrentTimeAsString()
    {
        DateTime now = DateTime.UtcNow;

        Holder currentHolder = _holder;
        if (currentHolder == null || Math.Abs(now.Ticks - currentHolder.TimeValue.Ticks) > TimeSpan.TicksPerSecond)
        {
            // value didn't exist or was out of date
            currentHolder = new Holder()
            {
                TimeValue = now,
                TimeValueAsString = now.ToString(rfc1123Pattern) // or whatever format
            };
            _holder = currentHolder;
        }

        return currentHolder.TimeValueAsString;
    }

    private sealed class Holder
    {
        internal DateTime TimeValue;
        internal string TimeValueAsString;
    }
}