[R] difftime problems with daylight saving time?

[ravi]

Hi,
I seem to be having problems in calculating the period between two times, one 
with and the other without daylight saving time. Consider the following command 
:

> as.numeric(difftime(as.POSIXct("2010-04-01 06:00:00 
>CEST"),as.POSIXct("2010-03-03 08:27:00 CET"),units="hours"))
[1] 692.55

No problem here. Strangely enough, I seem to be having problems in using the 
same command when applied to a data frame. Let me explain with the following 
code :
df<-structure(list(d1 = structure(c(1263279420, 1263885000, 1264492200,
1265094120, 1265784900, 1266391860, 1266908220, 1267515240, 1268117220,
1268722020, 1269327240, 1269841740, 1270532400, 1271138400, 1271833380,
1272349740, 1272953820, 1273470060, 1274075640, 1274768100, 1275285060,
1275976500, 1276494480, 1277100300, 1277703480, 1278309420, 1278913620,
NA, NA, NA, NA, 1282198020, 1282630920, 1283149380), class = c("POSIXt",
"POSIXct"), tzone = ""), d2 = structure(c(1263367500, 1263972540,
1264578600, 1265182740, 1265872260, 1266479460, 1266996720, 1267601220,
1268204100, 1268811960, 1269415200, 1269929340, 1270620300, 1271224800,
1271918520, 1272436020, 1273045800, 1273558740, 1274163720, 1274854140,
1275373620, 1276062900, 1276583400, 1277187060, 1277792100, 1278397800,
1279002840, NA, NA, NA, NA, 1282285380, 1282718460, 1283237940
), class = c("POSIXt", "POSIXct"), tzone = "")), .Names = c("d1",
"d2"), class = "data.frame", row.names = c(NA, -34L))
# get the first of corresponding month for d1
g1<-as.POSIXlt(df$d1,format="%Y-%m-%d %H:%M:%S")
g1$mday<-1;g1$hour<-6;g1$min<-0;
df$m1<-as.POSIXct(g1)
# period between d1 and m1
df$nh1<-with(df,round(as.numeric(difftime(d1,m1,units="hours")),1))
# get the first of next month for d2
g1$mon<-g1$mon+1
df$m2<-g1
#period between m2 and d2
df$nh2<-with(df,round(as.numeric(difftime(m2,d2,units="hours")),1))
str(df)

I get the following result :
> df
                    d1                  d2                  m1   
nh1                  m2   nh2
1  2010-01-12 07:57:00 2010-01-13 08:25:00 2010-01-01 06:00:00 265.9 2010-02-01 
06:00:00 453.6
2  2010-01-19 08:10:00 2010-01-20 08:29:00 2010-01-01 06:00:00 434.2 2010-02-01 
06:00:00 285.5
3  2010-01-26 08:50:00 2010-01-27 08:50:00 2010-01-01 06:00:00 602.8 2010-02-01 
06:00:00 117.2
4  2010-02-02 08:02:00 2010-02-03 08:39:00 2010-02-01 06:00:00  26.0 2010-03-01 
06:00:00 621.4
5  2010-02-10 07:55:00 2010-02-11 08:11:00 2010-02-01 06:00:00 217.9 2010-03-01 
06:00:00 429.8
6  2010-02-17 08:31:00 2010-02-18 08:51:00 2010-02-01 06:00:00 386.5 2010-03-01 
06:00:00 261.1
7  2010-02-23 07:57:00 2010-02-24 08:32:00 2010-02-01 06:00:00 530.0 2010-03-01 
06:00:00 117.5
8  2010-03-02 08:34:00 2010-03-03 08:27:00 2010-03-01 06:00:00  26.6 2010-04-01 
06:00:00    NA
9  2010-03-09 07:47:00 2010-03-10 07:55:00 2010-03-01 06:00:00 193.8 2010-04-01 
06:00:00    NA
10 2010-03-16 07:47:00 2010-03-17 08:46:00 2010-03-01 06:00:00 361.8 2010-04-01 
06:00:00    NA
11 2010-03-23 07:54:00 2010-03-24 08:20:00 2010-03-01 06:00:00 529.9 2010-04-01 
06:00:00    NA
12 2010-03-29 07:49:00 2010-03-30 08:09:00                <NA>    NA 2010-04-01 
06:00:00  45.9
13 2010-04-06 07:40:00 2010-04-07 08:05:00 2010-04-01 06:00:00 121.7 2010-05-01 
06:00:00 573.9
14 2010-04-13 08:00:00 2010-04-14 08:00:00 2010-04-01 06:00:00 290.0 2010-05-01 
06:00:00 406.0
15 2010-04-21 09:03:00 2010-04-22 08:42:00 2010-04-01 06:00:00 483.1 2010-05-01 
06:00:00 213.3
16 2010-04-27 08:29:00 2010-04-28 08:27:00 2010-04-01 06:00:00 626.5 2010-05-01 
06:00:00  69.5
17 2010-05-04 08:17:00 2010-05-05 09:50:00 2010-05-01 06:00:00  74.3 2010-06-01 
06:00:00 644.2
18 2010-05-10 07:41:00 2010-05-11 08:19:00 2010-05-01 06:00:00 217.7 2010-06-01 
06:00:00 501.7
19 2010-05-17 07:54:00 2010-05-18 08:22:00 2010-05-01 06:00:00 385.9 2010-06-01 
06:00:00 333.6
20 2010-05-25 08:15:00 2010-05-26 08:09:00 2010-05-01 06:00:00 578.2 2010-06-01 
06:00:00 141.8
21 2010-05-31 07:51:00 2010-06-01 08:27:00 2010-05-01 06:00:00 721.9 2010-06-01 
06:00:00  -2.5
22 2010-06-08 07:55:00 2010-06-09 07:55:00 2010-06-01 06:00:00 169.9 2010-07-01 
06:00:00 526.1
23 2010-06-14 07:48:00 2010-06-15 08:30:00 2010-06-01 06:00:00 313.8 2010-07-01 
06:00:00 381.5
24 2010-06-21 08:05:00 2010-06-22 08:11:00 2010-06-01 06:00:00 482.1 2010-07-01 
06:00:00 213.8
25 2010-06-28 07:38:00 2010-06-29 08:15:00 2010-06-01 06:00:00 649.6 2010-07-01 
06:00:00  45.8
26 2010-07-05 07:57:00 2010-07-06 08:30:00 2010-07-01 06:00:00  98.0 2010-08-01 
06:00:00 621.5
27 2010-07-12 07:47:00 2010-07-13 08:34:00 2010-07-01 06:00:00 265.8 2010-08-01 
06:00:00 453.4
28                <NA>                <NA>                <NA>    
NA                <NA>    NA
29                <NA>                <NA>                <NA>    
NA                <NA>    NA
30                <NA>                <NA>                <NA>    
NA                <NA>    NA
31                <NA>                <NA>                <NA>    
NA                <NA>    NA
32 2010-08-19 08:07:00 2010-08-20 08:23:00 2010-08-01 06:00:00 434.1 2010-09-01 
06:00:00 285.6
33 2010-08-24 08:22:00 2010-08-25 08:41:00 2010-08-01 06:00:00 554.4 2010-09-01 
06:00:00 165.3
34 2010-08-30 08:23:00 2010-08-31 08:59:00 2010-08-01 06:00:00 698.4 2010-09-01 
06:00:00  21.0

I would like to know the following :
1. Why do I get a NA result for m1[12]?
2. Why do I get NA for df$nh2[8:11]? If I run  df2$nh2[8] as a seperate command 
(shown at the very beginning of this posting), I get a result alright.
Do these difficulties have anything to do with one time object being CET and the 
other CEST? If yes, is there any workaround?

Thanking you,
Ravi