Choose the format for the job. ICS represents calendar objects. CSV represents rows of text. Both can describe a list of dates, but they preserve different information.
ICS is the better working copy for a calendar
An iCalendar file can carry event IDs, start and end dates, recurring rules, exception dates, changed occurrences, participants, reminders and time-zone definitions. If the job is migration, merge, split or backup, keeping the original ICS avoids flattening those relationships.
CSV is useful for review and reporting
A CSV is easy to search, sort and annotate in Excel or Google Sheets. CalPorter exports one row per stored VEVENT, including separate rows for modified instances. It includes recurrence fields as text, rather than claiming that one row is every occurrence of a repeating meeting.
A conversion is not always a round trip
Calendar applications expect their own CSV column names and date conventions. A generic spreadsheet export is not automatically a valid Google or Outlook import. Alarms, attachments, participants and provider-specific properties do not fit a small common CSV schema. Keep an ICS backup whenever these details matter.
Local time is not the same as UTC
A time ending in Z identifies a UTC instant. A date-time carrying TZID refers to a named zone. A floating local date-time has neither. Putting all three into a column labelled Date without preserving the distinction can silently shift an event.
Use a copy for spreadsheet inspection
Calendar titles can begin with characters that a spreadsheet treats as formulas. Our CSV export prefixes risky cells with an apostrophe. This protects inspection in common spreadsheet apps and deliberately changes those cells; the CSV is not a lossless backup.
Sources: RFC 5545, Google Calendar imports, and Apple Calendar import and export.