R help - Feb 2005 - how many 7th of the month is there between two dates

This is a eaeir way to ask my prior question:

I want to caculate how many an exact day of the month there is between
two dates.

For example; How many 7th of the month is there between "1998/12/17"
and "2000/1/7". To make the problem simple, the day of the month (7)
is the day in the 2nd date.

Omar Lakkis <uofiowa <at> gmail.com> writes:


: I want to caculate how many an exact day of the month there is between
: two dates.
: 
: For example; How many 7th of the month is there between "1998/12/17"
: and "2000/1/7". To make the problem simple, the day of the month (7)
: is the day in the 2nd date.

d1 <- as.Date("1998/12/17")
d2 <- as.Date("2000/1/7")

day <- function(x) as.numeric(format(x, "%d"))

sum(day(seq(from = d1, to = d2, by = "day")) == day(d2))

It is really easy using R's date and datetime classes:

st <- as.Date("1998-12-17")
en <- as.Date("2000-1-7")
st0 <- st
# Get the 7th of the month before
st0 <- as.POSIXlt(st); st0$mday <- 7; st0$mon <- st0$mon - 1
st0 <- as.Date(st0)
ll <- seq.Date(st0, en, by="month")
sum(ll > as.Date(st) & ll < en)

and use >= etc depending what you mean by `between'.

On Tue, 15 Feb 2005, Omar Lakkis wrote:
> This is a eaeir way to ask my prior question:
>
> I want to caculate how many an exact day of the month there is between
> two dates.
>
> For example; How many 7th of the month is there between
"1998/12/17"
> and "2000/1/7". To make the problem simple, the day of the month
(7)
> is the day in the 2nd date.
-- 
Brian D. Ripley,                  ripley at stats.ox.ac.uk
Professor of Applied Statistics,  http://www.stats.ox.ac.uk/~ripley/
University of Oxford,             Tel:  +44 1865 272861 (self)
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