1 files changed, 49 insertions, 0 deletions
diff --git a/unix/os/zgmtco.c b/unix/os/zgmtco.c
new file mode 100644
index 00000000..03fa5db8
--- /dev/null
+++ b/unix/os/zgmtco.c
@@ -0,0 +1,49 @@
+/* Copyright(c) 1986 Association of Universities for Research in Astronomy Inc.
+ */
+
+#include <stdio.h>
+#include <sys/types.h>
+#include <time.h>
+
+#define	import_kernel
+#define	import_knames
+#define import_spp
+#include <iraf.h>
+
+#define SECONDS_1970_TO_1980    315532800L
+
+
+/* ZGMTCO -- Return the correction, in seconds, from local standard time
+ * (clock time) to GMT.   GMT = LST + gmtco (seconds), or gmtco = GMT-LST.
+ */
+int
+ZGMTCO (
+  XINT	*gmtcor				/* seconds */
+)
+{
+	time_t gmt_to_lst(), ltime;
+
+	/* Given an input value of zero (biased by SECONDS_1970_TO_1980)
+	 * gmt_to_lst will return a negative value in seconds for a location
+	 * in the US (as an limiting test case).  We want to return the
+	 * correction to LST to get GMT, a positive value for the US, so
+	 * we need to negate this value.  gmt_to_lst will already have taken
+	 * daylight savings time into account.  Although we talk about the
+	 * US (as a test case) this relation will hold both east and west
+	 * of Greenwich.
+	 */
+
+	*gmtcor = -((XINT) gmt_to_lst ((time_t) SECONDS_1970_TO_1980));
+
+
+        /* Daylight saving time is not added to the output of gmt_to_lst()
+	 * since it assumes Jan 1.  Use the current date to determin if
+	 * DST is in effect.
+	 */
+
+	ltime = time(0);
+	if (localtime(&ltime)->tm_isdst)
+	    *gmtcor = *gmtcor - 60L * 60L;
+
+	return (XOK);
+}