diff --git a/respodns/ip_info.py b/respodns/ip_info.py
index 96b1ab0..a1270e4 100644
--- a/respodns/ip_info.py
+++ b/respodns/ip_info.py
@@ -7,7 +7,7 @@ from time import time
 CacheLine = namedtuple("CacheLine", ("time", "code"))
 header = ["ip", "time", "code"]
 
-one_month = 30 * 24 * 60 * 60  # in seconds
+one_month = 365.25 / 12 * 24 * 60 * 60  # in seconds
 encoding = "latin-1"
 
 cache_filepath = "ipinfo_cache.csv"