diff --git a/src/core/utils.jl b/src/core/utils.jl
index 25d1b1d69f1b21cf1a2035d459f64f2272435518..2e12c6649ab938e0d81a995b79a218d766178d3a 100644
--- a/src/core/utils.jl
+++ b/src/core/utils.jl
@@ -26,7 +26,7 @@ Stores a month and a day, and can be compared against normal dates.
 To save typing, a Tuple{Int64,Int64} is automatically converted to an
 AnnualDate, allowing this syntax: `nestingend::AnnualDate = (August, 15)`.
 """
-mutable struct AnnualDate
+struct AnnualDate
     month::Int64
     day::Int64