diff --git a/bits/20_jsutils.js b/bits/20_jsutils.js
index 97c0a14..40052fd 100644
--- a/bits/20_jsutils.js
+++ b/bits/20_jsutils.js
@@ -32,15 +32,21 @@ function evert_arr(obj/*:any*/)/*:EvertArrType*/ {
 }
 
 var basedate = new Date(1899, 11, 30, 0, 0, 0); // 2209161600000
-var dnthresh = basedate.getTime() + (new Date().getTimezoneOffset() - basedate.getTimezoneOffset()) * 60000;
 function datenum(v/*:Date*/, date1904/*:?boolean*/)/*:number*/ {
 	var epoch = v.getTime();
 	if(date1904) epoch -= 1462*24*60*60*1000;
+	var dnthresh = basedate.getTime() + (v.getTimezoneOffset() - basedate.getTimezoneOffset()) * 60000;
 	return (epoch - dnthresh) / (24 * 60 * 60 * 1000);
 }
+var refdate = new Date();
+var dnthresh = basedate.getTime() + (refdate.getTimezoneOffset() - basedate.getTimezoneOffset()) * 60000;
+var refoffset = refdate.getTimezoneOffset();
 function numdate(v/*:number*/)/*:Date*/ {
 	var out = new Date();
 	out.setTime(v * 24 * 60 * 60 * 1000 + dnthresh);
+	if (out.getTimezoneOffset() !== refoffset) {
+		out.setTime(out.getTime() + (out.getTimezoneOffset() - refoffset) * 60000);
+	}
 	return out;
 }
 
diff --git a/test.js b/test.js
index 93dfd69..23d3ea8 100644
--- a/test.js
+++ b/test.js
@@ -1422,10 +1422,10 @@ function seq(end/*:number*/, start/*:?number*/)/*:Array<number>*/ {
 }
 
 var basedate = new Date(1899, 11, 30, 0, 0, 0); // 2209161600000
-var dnthresh = basedate.getTime() + (new Date().getTimezoneOffset() - basedate.getTimezoneOffset()) * 60000;
 function datenum(v/*:Date*/, date1904/*:?boolean*/)/*:number*/ {
 	var epoch = v.getTime();
-	if(date1904) epoch += 1462*24*60*60*1000;
+	if(date1904) epoch -= 1462*24*60*60*1000;
+	var dnthresh = basedate.getTime() + (v.getTimezoneOffset() - basedate.getTimezoneOffset()) * 60000;
 	return (epoch - dnthresh) / (24 * 60 * 60 * 1000);
 }
 var good_pd_date = new Date('2017-02-19T19:06:09.000Z');
diff --git a/xlsx.flow.js b/xlsx.flow.js
index f6044bf..084f4c6 100644
--- a/xlsx.flow.js
+++ b/xlsx.flow.js
@@ -2751,15 +2751,21 @@ function evert_arr(obj/*:any*/)/*:EvertArrType*/ {
 }
 
 var basedate = new Date(1899, 11, 30, 0, 0, 0); // 2209161600000
-var dnthresh = basedate.getTime() + (new Date().getTimezoneOffset() - basedate.getTimezoneOffset()) * 60000;
 function datenum(v/*:Date*/, date1904/*:?boolean*/)/*:number*/ {
 	var epoch = v.getTime();
 	if(date1904) epoch -= 1462*24*60*60*1000;
+	var dnthresh = basedate.getTime() + (v.getTimezoneOffset() - basedate.getTimezoneOffset()) * 60000;
 	return (epoch - dnthresh) / (24 * 60 * 60 * 1000);
 }
+var refdate = new Date();
+var dnthresh = basedate.getTime() + (refdate.getTimezoneOffset() - basedate.getTimezoneOffset()) * 60000;
+var refoffset = refdate.getTimezoneOffset();
 function numdate(v/*:number*/)/*:Date*/ {
 	var out = new Date();
 	out.setTime(v * 24 * 60 * 60 * 1000 + dnthresh);
+	if (out.getTimezoneOffset() !== refoffset) {
+		out.setTime(out.getTime() + (out.getTimezoneOffset() - refoffset) * 60000);
+	}
 	return out;
 }
 
diff --git a/xlsx.js b/xlsx.js
index da15df5..8cfd42e 100644
--- a/xlsx.js
+++ b/xlsx.js
@@ -2672,15 +2672,21 @@ function evert_arr(obj) {
 }
 
 var basedate = new Date(1899, 11, 30, 0, 0, 0); // 2209161600000
-var dnthresh = basedate.getTime() + (new Date().getTimezoneOffset() - basedate.getTimezoneOffset()) * 60000;
 function datenum(v, date1904) {
 	var epoch = v.getTime();
 	if(date1904) epoch -= 1462*24*60*60*1000;
+	var dnthresh = basedate.getTime() + (v.getTimezoneOffset() - basedate.getTimezoneOffset()) * 60000;
 	return (epoch - dnthresh) / (24 * 60 * 60 * 1000);
 }
+var refdate = new Date();
+var dnthresh = basedate.getTime() + (refdate.getTimezoneOffset() - basedate.getTimezoneOffset()) * 60000;
+var refoffset = refdate.getTimezoneOffset();
 function numdate(v) {
 	var out = new Date();
 	out.setTime(v * 24 * 60 * 60 * 1000 + dnthresh);
+	if (out.getTimezoneOffset() !== refoffset) {
+		out.setTime(out.getTime() + (out.getTimezoneOffset() - refoffset) * 60000);
+	}
 	return out;
 }