[Hugh]

Thanks, a very nice digital tool for date conversion. 

Here's one of my départements conquis covers with an old-style date from after the revolution. It was mailed from  'Charleroy' (now Charleroi) to Mons on 7 Messidor Year X. Now in Belgium, both towns were then under French occupation and had been since 1794. Mons was the principal city of the new département 86 - lands taken from the Austrian Netherlands. 

There is a black ink hand-stamp P86P / CHARLEROY. The P86P means the post was pre-paid. This is also confirmed by the manuscript Franco in the upper right hand corner. In 1802 the rate for letters under 6g going less than 100km would have been 2 décimes.

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It was addressed: 
Au Citoyen
Dellainne, Directeur de
la Regie général d'affaires
rue des dominicains No. 30
à Mons

That is, To Citizen Dellainne, Director of General Admistration Policy, Number 30, Street of the Dominicans, in Mons.

The date was written at the top of the letter,  7 Messidor Year X:

   

Using the date converter we see that this is June 26, 1802. Cool.