[#353] Fix unit tests - strtotime returns incorrect month #544

Job	Run time
ci / check if job needs to run	4s
ci / prepare test matrix	19s
ci / MOODLE_310_STABLE - pgsql - php 7.2 - tool_excimer	3m 26s
ci / MOODLE_311_STABLE - mariadb - php 7.3 - tool_excimer	3m 13s
ci / MOODLE_35_STABLE - mariadb - php 7.1 - tool_excimer	3m 4s
ci / MOODLE_35_STABLE - mariadb - php 7.2 - tool_excimer	3m 33s
ci / MOODLE_35_STABLE - pgsql - php 7.1 - tool_excimer	2m 49s
ci / MOODLE_35_STABLE - pgsql - php 7.2 - tool_excimer	3m 17s
ci / MOODLE_36_STABLE - mariadb - php 7.1 - tool_excimer	3m 8s
ci / MOODLE_37_STABLE - pgsql - php 7.2 - tool_excimer	3m 10s
ci / MOODLE_38_STABLE - mariadb - php 7.3 - tool_excimer	3m 41s
ci / MOODLE_39_STABLE - mariadb - php 7.2 - tool_excimer	3m 29s
ci / MOODLE_39_STABLE - pgsql - php 7.2 - tool_excimer	3m 16s
ci / MOODLE_39_STABLE - mariadb - php 7.3 - tool_excimer	3m 26s
ci / MOODLE_39_STABLE - pgsql - php 7.3 - tool_excimer	3m 27s
ci / MOODLE_400_STABLE - pgsql - php 7.3 - tool_excimer	3m 22s
ci / MOODLE_401_STABLE - pgsql - php 7.4 - tool_excimer	3m 28s
ci / MOODLE_401_STABLE - pgsql - php 8.0 - tool_excimer	3m 33s
ci / MOODLE_401_STABLE - mariadb - php 8.0 - tool_excimer	3m 48s
ci / MOODLE_401_STABLE - mariadb - php 7.4 - tool_excimer	3m 20s
ci / Release to the Moodle plugin directory	0s
	1h 0m 53s

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