Data\Campaigns\Messages_en

[Wreford-Brown]

There should be two dates - the first one is the date (and time) that SH3 will start sending the message, the second is the date (and time) that it finishes sending the message.

You'll need to change both dates in order to get the message on the right day, with an appropriate time window (6 to 8 hours).

Do you have any JSGME or SH3 Commander mods that might overwrite the file? You can check by loading the game, alt-tabbing out of the game and going back into the messages_en file to see whether the changes are still there once the game has loaded.