I love Janto: they'll be sure to make the most of it.
"Happy New Year's Eve's Eve."
Ianto turned to see his boss walking towards him. "Jack, that's not a thing."
Jack's smile didn't falter. Nor did Jack attempt to argue, instead he pulled out some mistletoe and dangled it over Ianto's head. He kissed Ianto's nose before the Welshman could add that mistletoe after Christmas also wasn't a thing.
"I think I'll send the others home. It looks like a quiet night. But if you're not busy...?"
Ianto smiled slyly, got out his stopwatch and pressed the button. "I'll meet you in your office in ten."
Jack nearly asked Ianto what he was doing for New Years but then realized that time was of the essence. He decided to ask him later.