[R] Calculate the difference between dates

From: Tom Cohen <tom.cohen78_at_yahoo.se>
Date: Thu, 7 Feb 2008 12:39:11 +0100 (CET)


Dear list,   

I have two data columns (part of big data frame) containing dates presenting the dates when two measurements (M1 and M2) were taken. The data consists of 73 individuals divided in different groups. Each group was examined at different time points (see M1 date),but the measurements (M1 and M2) within each group should be taken in the same day but due to some practical issues, some M2-measurents were taken on another day. All individuals measured at the same day in column M1date belong to one group. For example, 24 measurements were taken 8/15/2005 (M1date column,one group of individual) but only 4 M2-mesurements were taken at the same day, the 24-2=20 M2-measurements were taken 2-65 days later within this group.   For analysis, I need to know how many days the M2-measurements were taken if they were not taken at the same day as M1-measurements within each group. Each of the different dates in column M1date would reflect the starting date for each group,for example 8/15/2005 is starting date for one group, and 8/8/2005 is a starting date for another group, so if M2-measurements wíthin group were taken for example 8/15/2005, then we say that they were taken at day 1, and if they were taken 8/16/2005 we say they were taken 2 days after the starting date for that group and so on. For group with starting date 8/8/2005, if the M2-measurements were taken at the same date, then these would be considered taken at day 1 and for those that were taken 8/9/2005, we say they were taken at day 2 for this group In the below data, I have manually calculated the number of days when M2-measurements were taken if they were not taken at
the starting day for some individuals (column Days). Is there any automatic ways to do this in R ?   

      M1date M2date Days

75 8/15/2005  8/15/2005  1
79 8/15/2005  8/16/2005  2
76 8/15/2005  8/15/2005  1
84 8/15/2005  8/16/2005  2
74 8/15/2005  8/15/2005  1
80 8/15/2005  8/16/2005  2
77 8/15/2005  8/15/2005  1
81 8/15/2005  8/16/2005  2
82 8/15/2005  8/16/2005  2
78 8/15/2005  8/16/2005  2
83 8/15/2005  8/16/2005  2
85 8/15/2005 10/17/2005  62
2  8/15/2005 10/19/2005  64
1  8/15/2005 10/18/2005  63
5  8/16/2005 10/19/2005  65
3  8/15/2005 10/19/2005  65
4  8/15/2005 10/19/2005  65
6  8/15/2005 10/19/2005  65
12  8/8/2005   8/9/2005   2
10  8/8/2005   8/9/2005   2
11  8/8/2005   8/9/2005   2
8   8/8/2005  11/7/2005  
7   8/8/2005  11/8/2005
9   8/8/2005   8/8/2005
29  8/8/2005 11/10/2005
25  8/8/2005  11/9/2005
30  8/8/2005 11/10/2005
28  8/8/2005  11/9/2005
32 8/15/2005 11/10/2005
33 8/15/2005 11/10/2005
31 8/15/2005 11/10/2005
24 8/15/2005  11/9/2005
26 8/15/2005  11/9/2005
27 8/15/2005  11/9/2005
14 7/31/2006 11/15/2006
18 7/31/2006 11/13/2006
13 7/31/2006 11/15/2006
16 7/31/2006 11/16/2006
20 7/31/2006 11/14/2006
17 7/31/2006 11/16/2006
19 7/31/2006 11/13/2006
37  8/7/2006   8/7/2006
39  8/7/2006   8/7/2006
42  8/7/2006  9/20/2006
49  8/7/2006  9/21/2006
52  8/7/2006  9/21/2006
50  8/7/2006  9/21/2006
47  8/7/2006  9/21/2006
38  8/7/2006   8/7/2006
45  8/7/2006  9/19/2006
43  8/7/2006  9/20/2006
48  8/7/2006  9/21/2006
36  8/7/2006   8/7/2006
44  8/7/2006  9/20/2006
46  8/7/2006  9/19/2006
51  8/7/2006  9/21/2006
41  8/7/2006   8/8/2006
40  8/7/2006   8/8/2006
68 7/31/2006  9/28/2006
59 7/31/2006   8/1/2006
69 7/31/2006  9/28/2006
71 7/31/2006  9/28/2006
58  8/1/2006  9/27/2006
60 7/31/2006   8/1/2006
70 7/31/2006  9/28/2006
66 7/31/2006  9/28/2006
67 7/31/2006  9/28/2006
72 7/31/2006  9/28/2006
64  8/1/2006   8/1/2006
61 7/31/2006   8/1/2006
62 7/31/2006   8/1/2006

63 7/31/2006 8/1/2006
65 7/31/2006 8/1/2006    

  Thanks for any help,
Tom        



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