[R] Incorrect Conversion of Datetime

[Ogbos Okike]

Dear Friends,
A sample of my data is:
98 05 01 02    8541
98 05 01 03    8548
98 05 01 04    8512
98 05 01 05    8541
98 05 01 06    8509
98 05 01 07    8472
98 05 01 08    8454
98 05 01 09    8461
98 05 01 10    8462
98 05 01 11    8475
98 05 01 12    8433
98 05 01 13    8479
98 05 01 14    8417
98 05 01 15    8463
98 05 01 16    8473
98 05 01 17    8450
98 05 01 18    8433
98 05 01 19    8437
98 05 01 20    8437
98 05 01 21    8438
98 05 01 22    8421
98 05 01 23    8420
98 05 02 00    8371
98 05 02 01    8338
98 05 02 02    8251
98 05 02 03    8204
98 05 02 04    8183
98 05 02 05    8231
98 05 02 06    8242
Columns 1, 2, 3, 4 and 5 stands for year, month, day , hour and count.

Using:
Sys.setenv( TZ="GMT" )


dta <- read.table("Ohr1may98", col.names = c("year", "month", "day",
"hour", "counts"))
dta$year <- with( dta, ifelse(year < 50, year + 2000, year + 1900))
dta$datetime <- with( dta, as.POSIXct(ISOdatetime(year, month,day,hour,0,0)))
a =  dta$datetime
I converted the datetime and plotted the graph of count vs a. The plot
was great but I have issues with the date.

The raw data is for some hours for Ist and second day of may 1998 as
is evident from the sample data. But the result of date stored in "a"
above shows:
> a
 [1] "1998-01-05 02:00:00 GMT" "1998-01-05 03:00:00 GMT"
 [3] "1998-01-05 04:00:00 GMT" "1998-01-05 05:00:00 GMT"
 [5] "1998-01-05 06:00:00 GMT" "1998-01-05 07:00:00 GMT"
 [7] "1998-01-05 08:00:00 GMT" "1998-01-05 09:00:00 GMT"
 [9] "1998-01-05 10:00:00 GMT" "1998-01-05 11:00:00 GMT"
[11] "1998-01-05 12:00:00 GMT" "1998-01-05 13:00:00 GMT"
[13] "1998-01-05 14:00:00 GMT" "1998-01-05 15:00:00 GMT"
[15] "1998-01-05 16:00:00 GMT" "1998-01-05 17:00:00 GMT"
[17] "1998-01-05 18:00:00 GMT" "1998-01-05 19:00:00 GMT"
[19] "1998-01-05 20:00:00 GMT" "1998-01-05 21:00:00 GMT"
[21] "1998-01-05 22:00:00 GMT" "1998-01-05 23:00:00 GMT"
[23] "1998-01-06 00:00:00 GMT" "1998-01-06 01:00:00 GMT"
[25] "1998-01-06 02:00:00 GMT" "1998-01-06 03:00:00 GMT"
[27] "1998-01-06 04:00:00 GMT" "1998-01-06 05:00:00 GMT"
[29] "1998-01-06 06:00:00 GMT"
This seems to suggest day 5 and 6 in January 1998 instead of day 1 and
2 in May of 1998.

I have spent some time trying to resolve this but I have not been successful.

I would be thankful if you could help me to check where I went astray.

Thank you.
Best wishes
Ogbos