[gnome-calendar] gcal-date-time-utils: Fix week start

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

commit 07f544b78d3c6c2808bdb9bbcf9d3927630033eb
Author: Evangelos Ribeiro Tzaras <devrtz fortysixandtwo eu>
Date:   Sun Jun 7 14:55:34 2020 +0200

    gcal-date-time-utils: Fix week start
    
    In locales where the first weekday is a monday, the previous code
    had a off by one error determining the week a sunday is in.
    The problem ultimately arose because in the
    `gcal_date_time_get_start_of_week()` function the variable
    `n_days_after_week_start` would become negative (-1 in this case) making
    the following `g_date_time_add_days (date, -n_days_after_week_start)`
    add 1 day resulting in `start_of_week` being set to the start of the
    following week.
    
    The solution was to make sure `n_days_after_week_start` would be
    positive by adding 7 to it. The variable is taken modulo 7 hence this
    should not introduce any bugs.
    
    Fixes #583

 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 733e4109..658bf6fd 100644
--- a/src/utils/gcal-date-time-utils.c
+++ b/src/utils/gcal-date-time-utils.c
@@ -84,7 +84,7 @@ gcal_date_time_get_start_of_week (GDateTime *date)
 
   first_weekday = get_first_weekday ();
   weekday = g_date_time_get_day_of_week (date) % 7;
-  n_days_after_week_start = (weekday - first_weekday) % 7;
+  n_days_after_week_start = (7 + weekday - first_weekday) % 7;
 
   start_of_week = g_date_time_add_days (date, -n_days_after_week_start);