[Matplotlib-users] Timezone response of date2num and mx2num differs

[Delbert D. F. <iq...@so...>]

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