If I understand your question, you can try this:
data$M1date <- as.Date(strptime(data$M1date, "%m/%d/%Y"))
data$M2date <- as.Date(strptime(data$M2date, "%m/%d/%Y"))
data$Days <- data$M2date - data$M1date
On 07/02/2008, Tom Cohen <tom.cohen78 at yahoo.se>
wrote:> 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 w?thin 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
>
>
>
> ---------------------------------
> G?r det l?ngsamt? Skaffa dig en snabbare bredbandsuppkoppling.
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Henrique Dallazuanna
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25? 25' 40" S 49? 16' 22" O