diff --git a/include/textproc.h b/include/textproc.h
index 12f28be01..d536a5511 100644
--- a/include/textproc.h
+++ b/include/textproc.h
@@ -79,6 +79,12 @@ namespace Anope
 	 */
 	extern CoreExport Anope::string FormatCTCP(const Anope::string &name, const Anope::string &body = "");
 
+	/** Formats a date/time as an IS0 8601 timestamp.
+	 * @param ts UNIX timestamp to format.
+	 * @param ms Number of milliseconds to format.
+	 */
+	extern CoreExport Anope::string FormatISO8601(time_t ts, unsigned long long ms);
+
 	/** Parses a CTCP message received from a client.
 	 * @param text The raw message to parse.
 	 * @param name The location to store the name of the CTCP.
diff --git a/modules/chanserv/cs_entrymsg.cpp b/modules/chanserv/cs_entrymsg.cpp
index 72e6799e9..da9d4839f 100644
--- a/modules/chanserv/cs_entrymsg.cpp
+++ b/modules/chanserv/cs_entrymsg.cpp
@@ -299,11 +299,7 @@ public:
 			{
 				Anope::map<Anope::string> tags;
 				if (timestamp)
-				{
-					char timebuf[32];
-					strftime(timebuf, sizeof(timebuf), "%Y-%m-%dT%H:%M:%S.000Z", gmtime(&message->when));
-					tags["time"] = timebuf;
-				}
+					tags["time"] = Anope::FormatISO8601(message->when, 0);
 
 				if (u->ShouldPrivmsg())
 					IRCD->SendContextPrivmsg(c->ci->WhoSends(), u, c, message->message, tags);
diff --git a/src/misc.cpp b/src/misc.cpp
index 175282ed2..66831df3b 100644
--- a/src/misc.cpp
+++ b/src/misc.cpp
@@ -1178,6 +1178,24 @@ Anope::string Anope::FormatCTCP(const Anope::string &name, const Anope::string &
 	return Anope::Format("\1%s %s\1", name.c_str(), value.c_str());
 }
 
+Anope::string Anope::FormatISO8601(time_t ts, unsigned long long ms)
+{
+	static time_t last_ts = -1;
+	static unsigned long long last_ms = -1;
+	static Anope::string timestamp;
+	if (ts != last_ts || ms != last_ms)
+	{
+		last_ts = ts;
+		last_ms = ms;
+		char timebuf[32];
+		const auto *tm = gmtime(&ts);
+		strftime(timebuf, sizeof(timebuf), "%Y-%m-%dT%H:%M:%S", tm);
+		timestamp = Anope::Format("%s.%03lldZ", timebuf, ms);
+	}
+	return timestamp;
+}
+
+
 bool Anope::ParseCTCP(const Anope::string &text, Anope::string &name, Anope::string &body)
 {
 	// According to draft-oakley-irc-ctcp-02 a valid CTCP must begin with SOH and