Burundi Visas are issued for a specific period of time. The entry date you write on your application, indicated on your itinerary, and/or if applicable your business letter will be the date your visa becomes valid for entry. You will not be able to enter before this date. For example your supporting documentation (application, itinerary and/or business letter) shows an entry date of January 1st- your visa would therefore be valid and allow you to enter Burundi no earlier than January 1st.
The Burundi visa is usually valid for 3 months from the date of issuance for either a single or multiple entries. The duration of stay is a maximum of 30 days per entry.