I tried the below code also before,but all the "resign_date" are invisiable no matter the date is "9000-12-31" or not,so I think the issue is the date format which I didnt know the correct code.

<t t-if="l.resign_date == '9000-12-31'">

I tried the below code will display the error of "="

<t t-if="l.resign_date = '9000-12-31'">

I am pretty sure provided code will work, another reason not working may this template/template portion replace using xpath in other module

addons/hr/model/hr_employee.base.py

Code:

resign_date = fields.Date(string="Resign Date", default="9000-12-31")

addons/hr/report/hr_employee_report.xml

PS:hr_employee_report.xml(Customize Report)

Code:

<td>

<t t-if="l.resign_date != datetime.datetime.strptime('9000-12-31','%Y-%m-%d')">

<span t-field="l.resign_date"/>

</t>

</td>

But in the printed report the date of '9000-12-31' still display

Even I tried the below code,still got the same result.

<t t-if="l.resign_date != 9000-12-31">

<t t-if="l.resign_date != '9000-12-31'">

can you try with a valid date as an example like '2015-12-15' or '2019-12-08'

Same issue