How do you calculate your cycles start date?

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Discussing with a friend today about when a cycle starts and realise can’t ask clinic on a weekend. What have your clinics said about cycle days?
Is it always just the calendar day, or if it’s say 10pm do they count the next day as day one for treatment? I guess there’s a big difference in starting at 5am one day and 11pm the same calendar day. Were you given cut offs?

I think broadly speaking if you start your period after 12pm ish, they count the next day as day 1. That's what my clinic has advised.

Yes my clinic also said that if your period starts after 12 then day 1 is classed as the following day.
My clinic also says it has to be full, red flow. Not brown/pink or spotting.

Thanks both that’s useful to know. Had always assumed it was just whenever you started in the day!

My clinic says day 1 is the day you get a full flow covering a pad (spotting/small amounts don’t count). So, with that in mind, I personally don’t get too caught up on time of the day, I just mark DAY 1 when I see that AF has arrived in its full force. If expecting to do a treatment, and AF arrives late in the evening, I would call my clinic early the following day to report that day 1 was the day before in the evening and then they know to make any adjustments necessary when to start shots/meds.

My clinic was also first full flow day. We just moved to a new clinic though, so I should probably check if they use the same calculation!