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    <p>The constructor for <a href="http://docs.python.org/2/library/datetime.html#datetime.datetime" rel="nofollow"><code>datetime</code></a> objects takes a year, month, and day, and optional hour, minute, etc. So, you're passing the day as the month, the hour as the day, and nothing as the hour.</p> <p>On top of that, <code>day</code> has to be a day of the month, not a day of the year, and <code>hour</code> has to be an integer (with separate minutes, seconds, and microseconds if appropriate).</p> <p>One easy way around this is to create a <code>datetime</code> with the start of the year, and add the days and hours on with <code>timedelta</code>.</p> <p>While we're at it, you can make the code a bit cleaner by iterating directly over the arrays instead of over <code>range(len(…))</code>.</p> <pre><code>for y, d, h in zip(year, day, hour): d0 = datetime.datetime(y, 1, 1, tzdata=pytz.UTC) dt.append(d0 + datetime.timedelta(days=d, hours=h)) </code></pre> <hr> <p>As a side note, I'd name the list variables <code>years</code>, <code>days</code>, <code>hours</code>, and <code>dts</code>, so I could name the individual iteration values <code>year</code>, <code>day</code>, <code>hour</code>, and <code>dt</code>. And, while we're at it, I'd write this as a helper function and a list comprehension rather than a <code>for</code> loop and a complicated body. And put the imports at the time. Like this:</p> <pre><code>import datetime import pytz years = [2012,2012,2012] days = [2,3,4] #day of year hours = [12,12,12] def make_date(year, day, hour, tzdata=pytz.UTC): dt = datetime.datetime(year, 1, 1, tzdata=tzdata) return dt + datetime.timedelta(days=day, hours=hour) dts = [make_date(year, day, hour) for year, day, hour in zip(years, days, hours)] </code></pre> <hr> <p>The above works for your original question, where <code>years</code> and <code>days</code> are lists of integers and <code>hours</code> is a list of floats (as in your description) or integers (as in your sample code). It will also work if <code>days</code> is a list of floats, but not <code>years</code>.</p> <p>However, from a comment, it sounds like these are actually numpy arrays of <code>int64</code> values. Although you call them floats, <code>int64</code> is not a floating-point type, it's an integer type. And you can convert an <code>int64</code> to a plain Python <code>int</code> with no loss of range or precision.</p> <p>So, because the <code>datetime</code> and <code>timedelta</code> constructors won't accept <code>int64</code> values, just convert each one into an <code>int</code>:</p> <pre><code>def make_date(year, day, hour, tzdata=pytz.UTC): dt = datetime.datetime(int(year), 1, 1, tzdata=tzdata) return dt + datetime.timedelta(days=int(day), hours=int(hour)) </code></pre> <p>If your hours were actually of a floating-point type as you originally claimed, like <code>float64</code>, you'd want to use <code>float(hour)</code> instead of <code>int(hour)</code>.</p> <hr> <p>One last thing: From your description, I guessed that your days were 0-based (and your hours, but that's not likely to be wrong). If your days are actually 1-based, you're obviously going to need to do <code>days=int(day)-1</code>.</p>
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    1. POConvert year, day of year and fractional hour to a real date in python
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    1. COHi Rob, thank you very much for your helpful comments. I do understand the constructor for datetime much better now. Thanks also for hints how to make the code shorter/better. For the given example it does work now.
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    2. CO@SabrinaNiebling: You're welcome, Tabitha. :) (I'm not Rob, but it's easy to miss who you're talking to on this site. Plus, being mistaken for someone as helpful as him isn't an insult. So, no harm done.)
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    3. COsorry for mixing up the names. I am still a bit confused by the way Stack Overflow works...
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