Why it's challenging to compare MoM in BiQuery?

Why month-over-month feels so natural?

MoM comparisons are familiar and expected. Everyone asks for them. Everyone trusts them. I used to as well.

What silently breaks the comparison

• Timezone mismatches
• Long user journeys
• Different behavioral patterns per month

Same SQL. Different underlying reality.

What a “month” really means in GA4

A month is not just a sum of days. It’s a behavioral slice with its own dynamics. BigQuery won’t warn you when that slice shifts.

How I compare periods safely now?

I use event_timestamp with timezone adjusting and treat each month as an individual chunk - with it's own specific.

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