diff --git a/docs/reference/units.md b/docs/reference/units.md
index af6bf3422..cf6cc0cb5 100644
--- a/docs/reference/units.md
+++ b/docs/reference/units.md
@@ -28,6 +28,8 @@ Pressure            | Pa, psi, atm
 Electricity and magnetism | ampere (A), coulomb (C), watt (W), volt (V), ohm, farad (F), weber (Wb), tesla (T), henry (H), siemens (S), electronvolt (eV)
 Binary              | bit (b), byte (B)
 
+Note: all time units are based on the Julian year, with one month being 1/12th of a Julian year, a year being one Julian year, a decade being 10 Julian years, a century being 100, and a millennium being 1000.
+
 Note that all relevant units can also be written in plural form, for example `5 meters` instead of `5 meter` or `10 seconds` instead of `10 second`. 
 
 Surface and volume units can alternatively be expressed in terms of length units raised to a power, for example `100 in^2` instead of `100 sqin`.