can anybody help me to find how to change the date to the exercise of GMAT in order to work anyway? I'm not able to solve this problem. Thanks
I assume you're referring to the Ex_MarsBPlane example script that comes with GMAT, and that you're trying to change the epoch date for the MAVEN spacecraft contained in that script.
The Ex_MarsBPlane example has hard-coded initial conditions that are based on the known relative geometry between Earth and Mars for the MAVEN mission's launch date (Nov 18 2013). It is meant to show how you would target a simple interplanetary trajectory correction maneuver (TCM) and Mars orbit insertion (MOI) maneuver if the initial conditions already took you near Mars.
If you change the date, you are changing the relative geometry and so the initial conditions won't result in a valid nominal trajectory anymore. This is why you are not able to re-target the TCM & MOI if you change the date. Note that if you change the date slightly, e.g. by 1 day, the targeter still converges. This is because the new nominal trajectory isn't that far off the original one. But any larger changes to date won't work with that script's setup.
Unfortunately this means that to change the date, you have to basically re-optimize the initial conditions, the same way as the actual launch provider and trajectory design team would. This requires knowledge of orbital mechanics and trajectory design & optimization, which is beyond the scope of this answer (without having a more specific question).