The maturity date of a note indicates the date when the note is due to be repaid to the investor along with any accrued interest, if it has not yet converted to equity.
In practice, in most situations, startup investors will not call for a note to be repaid at the maturity date, and will instead amend the note to extend the note’s maturity date, typically for a period of another year. This is because calling a note from a startup is usually self-defeating to the investor; the startup most likely will be bankrupted or at least adversely impacted by a called note, and by providing the startup with more time to reach a next milestone, the investor is getting an option on upside gain. Finally, calling a note and bankrupting a startup will most likely permanently tarnish the reputation of the investor, preventing them from gaining access to promising investment opportunities in the future as other investors and entrepreneurs alike avoid the investor. There are rare instances, for example when a startup has managed to reach profitability and has plenty of excess cash, where calling a note may not adversely impact a startup, but in those cases, an investor most likely has more to gain by enabling their note to convert to equity at a subsequent equity round or acquisition than to call their note.
There are some convertible notes that call for automatic conversion to equity at maturity date at a pre-defined price, but these are unusual.