What is a Week?

“Hey, big'un!" called Eupa.
"Do you know how hard it is not to call you small when you call me big?" he grumped.
"Don't give a shit, do it and die again," Eupa said without changing the dangerously cheerful note. "Call me mean, I'm in territory of okay with being mean as you are with being big."
"You think I like being big that much?" he grumped.
The teeth she showed when she smiled was an intentional display of power.
"Anyway," she continued, tossing and catching a knife and putting it away all in the same motion. He hated when she did that, and he checked to make sure she hadn't thrown one into his chest while she distracted him with the toss. It made her smile even wider.
"Stop picking on him, Eupa!" shouted Ro from up the hill ahead.
Eupa huffed air through pursed lips and pointed the knife at him. "Look, you need to lighten up, I don't get any better. In fact, this is literally my best fucking behavior. Okay? Work with me, here, I'm trying."
He didn't believe her, he'd seen her be nicer on accident. He was sure she couldn't be nicer on purpose, however, so she might have had a point.
"Anyway," she continued again, folding her hands behind her head to let him see them away from her belts. "What the fuck is a 'week'?"
It took Brotz several long moments to realize that while he expected them to have different ideas of weeks, he forgot that without a calendar at all, they would not have divided anything by anything except the moon phases he already knew about. They did call that a month, but they never called anything a week.
"I mean, how long is it? You obviously consider it a span of time, but I keep trying to guess it and I keep being wrong. And Ro and me have running bets."
Brotz looked at Ro to see her waiting expectantly. He was getting used to her face and its non-expressions, the way her face would go stern when she was looking for answers.
"Six days," he said after a moment, and Eupa cheered and Ro swore. "What were the bets?"
"I said five days, because Ro wouldn't let me guess 'between five and seven', and you'll say it for four days, and sometimes even three, but I don't know what the fuck "last week" means or how you figure it."
He looked to Ro with his brows lifted, hoping she'd tell him without having to ask.
"I guessed eight, because my range was between five and ten, and I know the local ones are closer to twelve and you've said that's too long. I assumed 'last week' was intended to mean within the span of time, so within eight days ago."
"That's right," he agreed. It was actually just a habit and generally intended to mean any time within the last ten days or so, without a calendar to reinforce his idea of what a week actually was, and what day it was. The local one was completely different, having two extra months and almost purely lunar, from what he could tell. Their weeks were nearly twice his, too, but he only knew that because of the scribes.
"What do you guys gamble?" he asked.
"Usually? Favors. Or getting out of favors. Sometimes snacks or treats, if we know the other's got something we want," Ro said. "Which is to say, when Eupa's got a snack or a treat that I want, and she wants me to do something for her, like find someone or something. Or she wants me to handle something that I've asked her to handle."
"Can you cancel each other's win with yours?"
"Yep. Keeps the number low enough to keep track of. We used to make it mandatory, only one of us could have any 'points' at a time, but then Eupa started stacking them because she's better at picking up patterns than I am."
"Gotta let me have something," Eupa laughed.