[gnome-calendar] date-time-utils: Fix weekday calculation

[Georges Basile Stavracas Neto <gbsneto src gnome org>]

commit 9644c30eee9723c57d10d97b11446224afc708fc
Author: Georges Basile Stavracas Neto <georges stavracas gmail com>
Date:   Sat Sep 14 11:14:01 2019 -0300

    date-time-utils: Fix weekday calculation
    
    GNOME Calendar needs to deal with two different
    weekday ranges:
    
     1. Sunday → 0 ... Saturday → 6
     2. Monday → 1 ... Sunday → 7 (used by GDateTime)
    
    When converting (2) into (1), the correct calculation
    is `weekday % 7`.
    
    However, gcal_date_time_get_start_of_week() was doing
    `weekday - 1` instead.
    
    Fix it and do the correct calculation instead.
    
    https://gitlab.gnome.org/GNOME/gnome-calendar/issues/451

 src/utils/gcal-date-time-utils.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)
---
diff --git a/src/utils/gcal-date-time-utils.c b/src/utils/gcal-date-time-utils.c
index 88b6725b..45ea57a6 100644
--- a/src/utils/gcal-date-time-utils.c
+++ b/src/utils/gcal-date-time-utils.c
@@ -83,7 +83,7 @@ gcal_date_time_get_start_of_week (GDateTime *date)
   g_assert (date != NULL);
 
   first_weekday = get_first_weekday ();
-  weekday = g_date_time_get_day_of_week (date) - 1;
+  weekday = g_date_time_get_day_of_week (date) % 7;
   n_days_after_week_start = (weekday - first_weekday) % 7;
 
   start_of_week = g_date_time_add_days (date, -n_days_after_week_start);