Use UTC timezone to determine month heading title

Summary:
The 2018-02-28 23:30 and 23:02 log entries are correctly considered
UTC by the JS code.

Yet, H2 month title uses local timezone, not UTC one. This triggers
a split of the title according browser local timezones:

• for local timezones in UTC or UTC-n, the correct month February is printed
• for local timezones in UTC+n, the +n 'March' is used instead.

This change fixes this bug asking the month in the UTC timezone.

Test Plan: Tested on https://infra.nasqueron.org/servers-log/

Reviewers: dereckson

Reviewed By: dereckson

Differential Revision: https://devcentral.nasqueron.org/D1370