diff --git a/vendor/ess/lib/sis8300drvbcm.c b/vendor/ess/lib/sis8300drvbcm.c
index 63a5892a4431e87cdc49dfb49fcacc613434e896..36cb001a40758626b2cc3140709c1b56a1a0de95 100644
--- a/vendor/ess/lib/sis8300drvbcm.c
+++ b/vendor/ess/lib/sis8300drvbcm.c
@@ -35,6 +35,8 @@
 #include <assert.h>
 #include <math.h>
 
+#include <epicsTime.h>
+
 #include <sis8300drv.h>
 #include <sis8300drv_utils.h>
 #include <sis8300_reg.h>
@@ -1869,7 +1871,7 @@ int sis8300drvbcm_set_timestamp(sis8300drv_usr *sisuser, unsigned int sec, unsig
         return ret;
     }
     // EPICS and POSIX Epoch differs in 20 year (631152000 seconds)
-    unsigned int value = sec + 631152000;
+    unsigned int value = sec + POSIX_TIME_AT_EPICS_EPOCH;
     ret = sis8300drv_reg_write(sisuser, SIS8300BCM_TIMESTAMP_HIGH_REG, value);
     if (ret) {
         return ret;
@@ -1889,7 +1891,7 @@ int sis8300drvbcm_get_timestamp(sis8300drv_usr *sisuser, unsigned int *sec, unsi
         return ret;
     }
     // EPICS and POSIX Epoch differs in 20 year (631152000 seconds)
-    *sec = value - 631152000;
+    *sec = value - POSIX_TIME_AT_EPICS_EPOCH;
     return ret;
 }