YTD Depreciation showing CR balance

hi,

we have placed an Asset in DEC-09 with Life period of 4 years,

Asset value : 1000

Depreciation for the periodof Dec-09 = 20.83.

In Jan-10 the same asset life period is extended by 4 more year i.e now the asset life is 8 years

for Jan Perdiod system shows the depreciation values as  -27.86 and the YTD deprecation for 20101 also shows -27.86. 

Now the question is why depreciation value  and YTD depreciation is showing credit amount.

What happens when we extend the life of the asset once it is placed in service and how is the YTD depreciation calculated when there is a change in the life of the asset in the running year.