[R] Extract year from date

MacQueen, Don
Mon, 09 Mar 2015 15:59:25 -0000

You need to solve step 1 (converting to Date class) before you can solve
step 2 (getting the year as a number).

In the data you are starting with, the first value in the Date column is
41163. You don't say whether that column is numeric or character, so
let's
try it both ways:

> as.Date('41163','%m/%d/%Y')
[1] NA

> as.Date(41163,'%m/%d/%Y')
Error in charToDate(x) :
  character string is not in a standard unambiguous format


Whatever it is, it is not formatted "%m/%d/%Y", so step 1 fails.


On the other hand, if you start with something that is in fact formatted
as a date:

> tmp <-  as.Date('3/2/2015','%m/%d/%Y')
> format(tmp,'%Y')
[1] "2015"
> as.numeric(format(tmp,'%Y'))
[1] 2015


you get the year as a number, using the method you tried.

So, what do 41163 and all the rest of the numbers in your Date column
actually represent???

Mark Sharp is probably correct in expecting that they represent the number
of days since some base date, and an easy way to see that would be to try
the core of step 1:

> as.Date(41163)
Error in as.Date.numeric(41163) : 'origin' must be supplied

It would be up to you, of course, to find out what the origin is. If
1/1/1900 is correct, then


> as.numeric( format( as.Date( 41163, origin='1900-1-1'),
'%Y'))
[1] 2012


(I personally prefer to use base R as much as possible)

-Don

--
Don MacQueen

Lawrence Livermore National Laboratory
7000 East Ave., L-627
Livermore, CA 94550
925-423-1062





On 3/7/15, 11:50 PM, "Steve Archambault" <archstevej at
gmail.com> wrote:

>Hi all,
>
>I am trying in vain to create a new object "Year" in my data frame
from
>existing Date data. I have tried many different approaches, but can't
seem
>to get it to work. Here is an example of some code I tried.
>
>date1<- as.Date(wells$Date,"%m/%d/%Y")
>wells$year<-as.numeric(format(date1, "%Y"))
>
>I am starting with data that looks like this.
>
>        ID  Date DepthtoWater_bgs test test2
>1  BC-0004 41163           260.60    3     1
>2  BC-0004 41255           261.65    4     2
>3  BC-0003 41345           166.58    5     3
>4  BC-0002 41351           317.85    6     4
>5  BC-0004 41355           262.15    7     5
>6  BC-0003 41438           167.55    8     6
>7  BC-0004 41438           265.45    9     7
>8  BC-0002 41443           317.25   10     8
>9  BC-0002 41521           321.25   11     9
>10 BC-0003 41522           168.65   12    10
>11 BC-0004 41522           266.15   13    11
>12 BC-0003 41627           168.95   14    12
>13 BC-0004 41627           265.25   15    13
>14 BC-0002 41634           312.31   16    14
>15 BC-0003 41703           169.25   17    15
>16 BC-0004 41703           265.05   18    16
>17 BC-0002 41710           313.01   19    17
>18 BC-0003 41795           168.85   20    18
>19 BC-0004 41795           266.95   21    19
>20 BC-0002 41801           330.41   22    20
>21 BC-0003 41905           169.75   23    21
>22 BC-0004 41905           267.75   24    22
>23 BC-0002 41906           321.01   25    23
>
>Any help would be greatly appreciated!
>
>-Steve
>Sent from my iPhone
>
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