yes sadly. i always try to do mine the day before they are due as i once had a vet that told me i had to restart if i did it exactly a year to the day of the previous one. I now dont take the risk!
Tetanus is not subject to the same stringent rules as 'flu. Your horse will be fine to have the tet booster a week or 2 after the dates.
'Flu however is a different story if you plan to do any competing of any kind - in that case you must adhere to jockey club or FEI rules and for them the dates are vitally important - the anniversary is acceptable but not a day later.