[Proj] large difference while projecting utm to utm back and forth

[Christoph Dohmen]

Dear list,

I know, I should not do it but I have to :-(
Given is a point 600000 east and 8260000 north in utm zone 35.
I have to project this location to utm zone 30.
Doing some calculations I have to project this value back from 
zone 30 to zone 35.
But I get differences of about 3km east and more than 200m north.

Those are the steps I made using cs2cs:
cs2cs +proj=utm +zone=35 +datum=WGS84 +to +proj=utm +zone=30 +datum=WGS84
600000 8260000
1451785.04      8532758.68 0.00

and the way back:
cs2cs +proj=utm +zone=30 +datum=WGS84 +to +proj=utm +zone=35 +datum=WGS84
1451785.04      8532758.68
597050.49       8260246.21 0.00

Can anyone describe a way to achive the startvalues?

Best regards

Christoph

P.S.
I use proj 4.7.1 and the latest from svn.
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