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Dwa tygodnie to jeśli dobrze liczę jest "fourteen nights". A szczegóły tutaj : http://en.wikipedia.org/wiki/Fortnight

Fortnight is a unit of time equal to fourteen days, or two weeks. The word derives from the Old English fēowertyne niht, meaning "fourteen nights".[1][2] Fortnight and fortnightly are commonly used words in Britain and many Commonwealth countries such as Australia, India, New Zealand, and Pakistan, where many wages and salaries and most social security benefits are paid on a fortnightly basis.[3] The word is rarely used in North America, except regionally in Canada and in insular traditional communities (e.g. Amish) in the United States. American and Canadian payroll systems may use the term biweekly in reference to pay periods every two weeks. Neither term should be confused with semimonthly (in one year there are 26 fortnightly or biweekly versus 24 semimonthly pay periods).

1. The Concise Oxford Dictionary, 5th Edition, 1964, p. 480
2 Senight, sennight or se'night (seven-night), an old word for the week, was still in use in the early nineteenth century, to judge from Jane Austen's letters.
3 "Australian Government - How much Disability Support Pension do I get?". Archived from the original on 2008-04-16. Retrieved 2008-05-22.
4 Littmann, Mark; Fred Espenak, Ken Willcox (2008). Totality: Eclipses of the Sun. Oxford University Press. ISBN 0-19-953209-5.
5. Synodic Month definition Eric W. Weisstein