diff --git a/cxx/mount_astrom.h b/cxx/mount_astrom.h
index e1f56cb..d6016a2 100644
--- a/cxx/mount_astrom.h
+++ b/cxx/mount_astrom.h
@@ -22,10 +22,75 @@ concept mcc_scalar_or_simd_c = xsimd::is_batch<T>::value || std::is_arithmetic_v
 namespace mcc::astro
 {
 
-// modified Julian date
-template <traits::mcc_scalar_or_simd_c T>
-int mcc_julday(const std::chrono::utc_clock::time_point& greg_time, T& mjd)
+// modified Julian date (based on ERFA eraCal2jd)
+// template <traits::mcc_scalar_or_simd_c T>
+static int mcc_julday(const std::chrono::system_clock::time_point& start_time,
+                      const std::chrono::system_clock::duration& step,
+                      std::vector<double, xsimd::default_allocator<double>>& mjd)
 {
+    if (mjd.empty()) {
+        return -100;
+    }
+
+    using namespace std::literals::chrono_literals;
+
+    auto dd = std::chrono::floor<std::chrono::days>(start_time);
+    std::chrono::year_month_day ymd{dd};
+
+    static constexpr std::chrono::year MIN_YEAR = -4799y;
+
+    if (ymd.year() < MIN_YEAR) {
+        return -1;
+    }
+    if (!ymd.month().ok()) {
+        return -2;
+    }
+
+    // my = (im - 14) / 12;
+    // iypmy = (long) (iy + my);
+
+    int64_t my = -(14 - (unsigned)ymd.month()) / 12;
+    int64_t iypmy = (int)ymd.year() + my;
+
+    // (1461L * (iypmy + 4800L)) / 4L
+    //     + (367L * (long) (im - 2 - 12 * my)) / 12L
+    //     - (3L * ((iypmy + 4900L) / 100L)) / 4L
+    //     + (long) id - 2432076L
+
+    // integer part of result MJD
+    int64_t mjd_int = 1461LL * (iypmy + 480LL) / 4LL +
+                      (367LL * ((int64_t)(unsigned)ymd.month() - 2LL - 12LL * my)) / 12LL -
+                      (3LL * (iypmy + 4900LL) / 100LL) / 4LL + (int64_t)(unsigned)ymd.day() - 2432076LL;
+
+    constexpr double nanosec = 1.0 / 24.0 / 3600.0 / 1.0E-9;  // 1 nanosecond in days
+
+    double mjd_float = static_cast<double>(mjd_int) +
+                       std::chrono::duration_cast<std::chrono::nanoseconds>(start_time - dd).count() * nanosec;
+    double d_step = std::chrono::duration_cast<std::chrono::nanoseconds>(step).count() * nanosec;
+
+    constexpr size_t reg_size = xsimd::batch<double>::size;
+    size_t vec_size = mjd.size() - mjd.size() % reg_size;
+
+    xsimd::batch<double> res_reg{mjd_float};
+    xsimd::batch<double> step_reg = [d_step]<size_t... Is>(std::index_sequence<Is...>) {
+        return xsimd::batch<double>{(Is * d_step)...};
+    }(std::make_index_sequence<reg_size>{});
+
+    // vectorized part
+    size_t i = 0;
+    for (; i < vec_size; i += vec_size) {
+        res_reg += step_reg;
+
+        res_reg.store_aligned(mjd.data() + i);
+    }
+
+    // scalar part
+    for (size_t j = i; j < mjd.size(); ++j) {
+        mjd[j] = mjd_float + j * d_step;
+    }
+
+
+    return 0;
 }
 
 template <traits::mcc_scalar_or_simd_c T, typename CT, typename DT>