日期之间的小时数

Question

I have a netCDF file and I am attempting to identify the first date in the file and the 'base date'. The file contains monthly data. My notes, which are fairly old, indicate the first date is January 1, 1948.

The following R code gives the first date as 17067072:

library(ncdf)
library(chron)
my.data <- open.ncdf('my.netCDF.nc')
x = get.var.ncdf(my.data, "lon" )
y = get.var.ncdf(my.data, "lat" )
z = get.var.ncdf(my.data, "time")
z[1:5]

#  [1] 17067072 17067816 17068512 17069256 17069976

I downloaded an application called ncdump.exe and after typing the following line in the Windows command window:

C:\Users\Mark W Miller\ncdump>ncdump -h my.netCDF.nc

I learned that the base date is:

time:units = "hours since 1-1-1 00:00:0.0" ;

This same base data is obtained in R using:

att.get.ncdf(my.data,"time","units")$value
[1] "hours since 1-1-1 00:00:0.0"

I tried to verify that with the following R code:

date1 <- as.Date("01/01/0001", "%m/%d/%Y")
date1
# [1] "0001-01-01"

date2 <- as.Date("01/01/1948", "%m/%d/%Y")
date2
# [1] "1948-01-01"

period <- as.Date(date1:date2, origin = "00-01-01")

hours <- 24 * (length(period)-1)
hours
# [1] 17067024

There is a difference of 48 hours between the number in z[1] and the number returned by the R code immediately above:

17067072 - 17067024
[1] 48

Where is my error? Since the netCDF file contains monthly data I doubt the first date is January 3, 1948. The website from which I downloaded the data does not offer the option of selecting the day within month.

The application ncdump.exe can be downloaded from here:

http://www.narccap.ucar.edu/data/ascii-howto.html

If I can figure out how to subset the netCDF file I might upload the smaller file somewhere.

Thank you for any advice.

Answer 1

Have you looked at your period vector? when I looked at the first few and last few values the year comes out as something that does not make sense. Possibly something is messed up in one of the conversions.

Also note that same computer programs treat 1900 as a leap year even though it was not, a difference between 2 programs on that factor could account for 24 of the hours in your difference.