title: Change two separate columns of month and year into one for date in R
body: <p>I am really new at using R and am having difficulty changing two columns into one for time. I have Months in 1-12 and years in 1999 etc. format. I am looking to change into one Date which R will recognise as a date. I tried using strptime but as I do not have a day this caused it to automatically use the current date instead - as date of the month isn't specified does this matter?</p>
  
  <p>Thanks in advance</p>
  
  <pre><code>&gt; head(tthm.data)
  WSZ_Code Treatment_Code Year Month TTHM CL2_FREE    BrO3 Colour  PH  TURB
  1         2              3 1996     1 30.7     0.35 0.00030   0.75 7.4 0.055
  2         6              1 1996     2 24.8     0.25 0.00055   0.75 6.9 0.200
  3         7              4 1996     2 60.4     0.05 0.00055   0.75 7.1 0.055
  6         5              2 1996     3 40.3     0.15 0.00140   2.00 7.7 0.055
  11        4              1 1996     3 46.5     0.25 0.00055   1.90 7.4 0.150
  14        2              3 1996     3 28.4     0.25 0.00055   1.80 7.4 0.055
  </code></pre>