Timezone response of date2num and mx2num differs

John, thanks for the updated mx2num but I still found something
puzzling: date2num takes the timezone from the matplotlibrc file
but mx2num must take it from some info on my machine. There is a
consistent seven hour difference between the two even though the
datetime instances give exactly the same time.

I need to point out that in my applications I have no interest in
the time zone and I want it always treated as if I am
on the Greenwich Meridian. Here's why: My simulation software
has used what is essentially the modified Julian Day number
from astronomy with a zero point around 1878 or so. I just add
the fraction of the day to it. This works
fine in my applications because there is no data to run the model
earlier than about the 1900! Also I have users in different time
zones sending me test cases and it would be a mess to have to
somehow always adjust for time zone. Thus I have effectively
computed the day numbers relative to the local time zone always.

Another benefit is that there is a fixed offset between my
time scale and the one used in plot_dates. Makes life a little
simpler.

Questions:

1. Should mx2num use a different time zone than does date2num?

2. If so, how do I get mx2num at its invocation only, to use
   the time zone in the matplotlibrc file? I assume I have
   to use the set_tzinfo method right before mx2num is invoked
   and then use set_tzinfo again right after to return to
   what it was.

Heres an example test file:

from matplotlib.dates import mx2num, date2num
import datetime
import mx.DateTime

for ihr in xrange(24):
    yr = 2003
    mn = 10
    day = 20
    imin = 30
    second = 0.0
    isec = 0

    dt = mx.DateTime.DateTime(yr, mn, day, ihr, imin, second)
    dt2 = datetime.datetime(yr, mn, day, ihr, imin, isec)
    print 'hour=', ihr
    print 'mx dt=',dt
    print 'dt2=', dt2
    diff = mx2num(dt) - date2num(dt2)
    print 'mx2num(dt)=',mx2num(dt)
    print 'date2num(dt2) =', date2num(dt2)
    print 'difference=', diff*24.0
    print ' '

Here's some output at my time zone:

hour= 0
mx dt= 2003-10-20 00:30:00.00
dt2= 2003-10-20 00:30:00
mx2num(dt)= 731508.3125
date2num(dt2) = 731508.020833
difference= 6.99999999907

hour= 1
mx dt= 2003-10-20 01:30:00.00
dt2= 2003-10-20 01:30:00
mx2num(dt)= 731508.354167
date2num(dt2) = 731508.0625
difference= 6.99999999907

hour= 2
mx dt= 2003-10-20 02:30:00.00
dt2= 2003-10-20 02:30:00
mx2num(dt)= 731508.395833
date2num(dt2) = 731508.104167
difference= 7.00000000186

hour= 3
mx dt= 2003-10-20 03:30:00.00
dt2= 2003-10-20 03:30:00
mx2num(dt)= 731508.4375
date2num(dt2) = 731508.145833
difference= 6.99999999907

hour= 4
mx dt= 2003-10-20 04:30:00.00
dt2= 2003-10-20 04:30:00
mx2num(dt)= 731508.479167
date2num(dt2) = 731508.1875
difference= 6.99999999907

hour= 5
mx dt= 2003-10-20 05:30:00.00
dt2= 2003-10-20 05:30:00
mx2num(dt)= 731508.520833
date2num(dt2) = 731508.229167
difference= 7.00000000186

                                Delbert

I just did another test. mx2num fails for dates prior to 1970 because
it uses the magical Unix seconds from epoch. We need mx2num to work
for the whole range of dates for which date2num works-at least that
is my vote.
                                  Delbert

···

On Wednesday 02 February 2005 10:10 am, Delbert D. Franz wrote:

John, thanks for the updated mx2num but I still found something
puzzling: date2num takes the timezone from the matplotlibrc file
but mx2num must take it from some info on my machine. There is a
consistent seven hour difference between the two even though the
datetime instances give exactly the same time.

I need to point out that in my applications I have no interest in
the time zone and I want it always treated as if I am
on the Greenwich Meridian. Here's why: My simulation software
has used what is essentially the modified Julian Day number
from astronomy with a zero point around 1878 or so. I just add
the fraction of the day to it. This works
fine in my applications because there is no data to run the model
earlier than about the 1900! Also I have users in different time
zones sending me test cases and it would be a mess to have to
somehow always adjust for time zone. Thus I have effectively
computed the day numbers relative to the local time zone always.

Another benefit is that there is a fixed offset between my
time scale and the one used in plot_dates. Makes life a little
simpler.

Questions:

1. Should mx2num use a different time zone than does date2num?

2. If so, how do I get mx2num at its invocation only, to use
   the time zone in the matplotlibrc file? I assume I have
   to use the set_tzinfo method right before mx2num is invoked
   and then use set_tzinfo again right after to return to
   what it was.

Heres an example test file:

from matplotlib.dates import mx2num, date2num
import datetime
import mx.DateTime

for ihr in xrange(24):
    yr = 2003
    mn = 10
    day = 20
    imin = 30
    second = 0.0
    isec = 0

    dt = mx.DateTime.DateTime(yr, mn, day, ihr, imin, second)
    dt2 = datetime.datetime(yr, mn, day, ihr, imin, isec)
    print 'hour=', ihr
    print 'mx dt=',dt
    print 'dt2=', dt2
    diff = mx2num(dt) - date2num(dt2)
    print 'mx2num(dt)=',mx2num(dt)
    print 'date2num(dt2) =', date2num(dt2)
    print 'difference=', diff*24.0
    print ' '

Here's some output at my time zone:

hour= 0
mx dt= 2003-10-20 00:30:00.00
dt2= 2003-10-20 00:30:00
mx2num(dt)= 731508.3125
date2num(dt2) = 731508.020833
difference= 6.99999999907

hour= 1
mx dt= 2003-10-20 01:30:00.00
dt2= 2003-10-20 01:30:00
mx2num(dt)= 731508.354167
date2num(dt2) = 731508.0625
difference= 6.99999999907

hour= 2
mx dt= 2003-10-20 02:30:00.00
dt2= 2003-10-20 02:30:00
mx2num(dt)= 731508.395833
date2num(dt2) = 731508.104167
difference= 7.00000000186

hour= 3
mx dt= 2003-10-20 03:30:00.00
dt2= 2003-10-20 03:30:00
mx2num(dt)= 731508.4375
date2num(dt2) = 731508.145833
difference= 6.99999999907

hour= 4
mx dt= 2003-10-20 04:30:00.00
dt2= 2003-10-20 04:30:00
mx2num(dt)= 731508.479167
date2num(dt2) = 731508.1875
difference= 6.99999999907

hour= 5
mx dt= 2003-10-20 05:30:00.00
dt2= 2003-10-20 05:30:00
mx2num(dt)= 731508.520833
date2num(dt2) = 731508.229167
difference= 7.00000000186

                                Delbert

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