Alright, in that case I managed to fix my wallet in time so here's a signed messaged from it, although it's not in BEGIN BITCOIN SIGNED MESSAGE format because I couldn't get Electrum to generate them that way.
IKnnQ1JsAPUk3NfqkT03/n1Kpekgeo8lFV0gaKLlXAM/Lii98FkFS93ubUWMARn0BFMiN7OsivlzoD1HOTNSZes=
OK, quoted, https://jhoenicke.github.io/brainwallet.github.io/#verify?verifieD
Format of the message :
Proof that I own 142eRsdiiteSL6pbBAV2NXRRZuCwz94pYP
-----BEGIN SIGNATURE-----
142eRsdiiteSL6pbBAV2NXRRZuCwz94pYP
IKnnQ1JsAPUk3NfqkT03/n1Kpekgeo8lFV0gaKLlXAM/Lii98FkFS93ubUWMARn0BFMiN7OsivlzoD1HOTNSZes=
-----END BITCOIN SIGNED MESSAGE-----
There is no date, now everyone can copy this one and use it pretending that it's you.
I suggest use this format :
Today is May 12, 2020.