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Editorial Team
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Asked: 2026-06-18T01:49:22+00:00

I’m parsing weekly time series data from a source that shows dates using a

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I’m parsing weekly time series data from a source that shows dates using a year and week number. However, when trying to use Python’s datetime.strptime function to turn these into YYYY-MM-DD dates, two different week numbers sometimes evaluate to the same date, when I know that they should not. The weekly data is for the week ending on a Friday. For example:

datetime.strptime("1998-5-53", "%Y-%w-%U")
Out[43]: datetime.datetime(1999, 1, 8, 0, 0)

datetime.strptime("1999-5-01", "%Y-%w-%U")
Out[44]: datetime.datetime(1999, 1, 8, 0, 0)

The underlying data from the European Central Bank; an example series is here.

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1 Answer

  1. Editorial Team

    1998 did not have 53 weeks; December 31st is week 52:

    >>> datetime.strptime("1998-4-52", "%Y-%w-%U")
datetime.datetime(1998, 12, 31, 0, 0)

    Note that the documentation for %U states:

    Week number of the year (Sunday as the first day of the week) as a decimal number [00,53]. All days in a new year preceding the first Sunday are considered to be in week 0.

    It could be that your data source sees the days preceding the first sunday as week 1 instead.

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