title: Combine year, month and day in Python to create a date
body: <p>I have a dataframe that consists of separate columns for year, month and day. I tried to combine these individual columns into one date using:</p>
  
  <pre><code>df['myDt']=pd.to_datetime(df[['year','month','day']])
  </code></pre>
  
  <p>only to get the following error: "to assemble mappings requires at least that [year, month, day] be specified: [day,month,year] is missing". Not sure what this means....I'm already supplying the relevant columns. 
  On checking the datatypes, I found that they Year, Month and Day columns are int64. Would that be causing an issue? 
  Thanks,
  Chet</p>
  
  <p>Thank you all for posting. As suggested, I'm posting the sample data set first:<code>
             Value        mm  yy     dd
  Date<br>
  2018-11-30  88.550067   11  2018    1
  2018-12-31  88.906290   12  2018    1
  2019-01-31  88.723000   1   2019    1
  2019-02-28  89.509179   2   2019    1
  2019-03-31  90.049161   3   2019    1
  2019-04-30  90.523100   4   2019    1
  2019-05-31  90.102484   5   2019    1
  2019-06-30  91.179400   6   2019    1
  2019-07-31  90.963570   7   2019    1
  2019-08-31  92.159170   8   2019    1</code></p>
  
  <p>The data source is:<a href="https://www.quandl.com/data/EIA/STEO_NGPRPUS_M" rel="noreferrer">https://www.quandl.com/data/EIA/STEO_NGPRPUS_M</a>
  I imported the data as follows:
  1. import quandl (used conda install first)
  2. Used Quandl's Python code:</p>
  
  <p>data=quandl.get("EIA/STEO_NGPRPUS_M", authtoken="TOKEN","2005-01-01","2005-12-31")
  4. Just to note, the original data comes only with the Value column, and DateTime as index. I extracted and created the mm,yy and dd columns (month, year, and dd is a column vector set to 1)
  All I'm trying to do is create another column called "first of the month" - so for each day of each month, the column will just show "MM/YY/1". I'm going to try out all the suggestions below shortly and get back to you guys. Thanks!!</p>