Yes, you have a maximum of 18 months to complete your myTax questionnaire. After this period, the data already entered will be deleted and, if necessary, you will have to start the questionnaire again for the year in question.
However, we would like to remind you below of the deadlines imposed by the Luxembourg tax authority (“Administration des Contributions Directes”) for submitting your return:
In general, it invites you to prepare your annual tax return before 31 March. That said, this date is merely a recommendation, the actual deadline being 31 December of year “n+1” (for income in year “n”).
After this date, the Luxembourg tax authority will no longer take any tax deductions into account, whether from special expenses, extraordinary expenses, etc.
Specifically, for married resident or non-resident taxpayers who wish to change their tax method (for example, to switch from joint to individual taxation or vice versa): the deadline is 31 March.