Hi, stata users

I want some explanation about "offset variable"
My dataset below is a unbalanced panel data,
What I want to look for is level of noise exposure has impact on occurence of disease.

I heard that offset variable can be used to take account the exposure time in the analysis...so,
when I run the xtgee with offset variable "year" (which is the year that subject has
participated the survey), the result come out significant(the exposure makes possibility of outcome high).
but when I don't consider the year as offset variable, it is insignificant..

Am I wrong to enter the "year" as offset variable??

thank you in advance..

Code:
* Example generated by -dataex-. To install: ssc install dataex
clear
input double(P_no R_time atopyDx1y year age sex) byte noise_q double Noisemodel
1 1 1 2013  9 2 1  42.81393064795522
1 2 1 2015 11 2 1  42.20467563905939
2 1 1 2013  9 2 1 43.211582234526695
2 2 1 2015 11 2 1  42.78365005924527
3 1 0 2013  7 2 3 50.711780353822846
end
-----------------


xtset P_no year
panel variable: P_no (unbalanced)
time variable: year, 2009 to 2018, but with gaps
delta: 1 unit

. xtgee atopyDx1y Noisemodel, family(binomial 1) link(logit) offset(year) corr(ex
> changeable) vce(robust)

Iteration 1: tolerance = .00693255
Iteration 2: tolerance = .00191436
Iteration 3: tolerance = .00073082
Iteration 4: tolerance = .00029676
Iteration 5: tolerance = .00012216
Iteration 6: tolerance = .00005058
Iteration 7: tolerance = .000021
Iteration 8: tolerance = 8.726e-06
Iteration 9: tolerance = 3.629e-06
Iteration 10: tolerance = 1.509e-06
Iteration 11: tolerance = 6.278e-07

GEE population-averaged model Number of obs = 10976
Group variable: P_no Number of groups = 4731
Link: logit Obs per group: min = 1
Family: binomial avg = 2.3
Correlation: exchangeable max = 4
Wald chi2(1) = 631.30
Scale parameter: 1 Prob > chi2 = 0.0000

(Std. Err. adjusted for clustering on P_no)
------------------------------------------------------------------------------
| Semirobust
atopyDx1y | Coef. Std. Err. z P>|z| [95% Conf. Interval]
-------------+----------------------------------------------------------------
Noisemodel | .189584 .0075454 25.13 0.000 .1747953 .2043728
_cons | -2024.045 .3792067 -5337.58 0.000 -2024.788 -2023.302
year | 1 (offset)
------------------------------------------------------------------------------

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