Editing Julian day (section)

=== Finding day of week given Julian day number ===
Because a Julian day starts at noon while a civil day starts at midnight, the Julian day number needs to be adjusted to find the day of week: for a point in time in a given Julian day after midnight UT and before 12:00&nbsp;UT, add 1 or use the JDN of the next afternoon.

The US day of the [[week]] '''W1''' (for an afternoon or evening UT) can be determined from the Julian Day Number '''J''' with the expression:
{{block indent|1='''W1''' = [[Modular arithmetic|mod]](''J'' + 1, 7)<ref>Richards 2013, pp. 592, 618.</ref>}}

{| class="wikitable"
|-style="text-align:center;"
!W1
| 0 || 1 || 2 || 3 || 4 || 5 || 6
|-
!Day of the week
|Sun||Mon||Tue||Wed||Thu||Fri||Sat
|}

If the moment in time is after midnight UT (and before 12:00&nbsp;UT), then one is already in the next day of the week.

The ISO day of the week '''W0''' can be determined from the Julian Day Number '''J''' with the expression:
{{block indent|1='''W0''' = mod (''J'', 7) + 1}}

{| class="wikitable"
|- style="text-align:center;"
!W0
| 1 || 2 || 3 || 4 || 5 || 6 || 7
|-
!Day of the week
|Mon||Tue||Wed||Thu||Fri||Sat||Sun
|}