TIME
Everything is relative
Traditional time
Today's basis is Coordinated Universal Time (UTC), which replaced Greenwich Mean Time(GMT). The military refers to UTC as Zulu time.
London:
- Winter: UTC/GMT
- summer (last sunday of March through last sunday of October): BST (British Summer Time), UTC/GMT+1
US has six timezones:
- Eastern (R - Romeo)
- Central (S - Sierra)
- Mountain (T - Tango)
- Pacific (U - Uniform)
- Alaska (V - Victor)
- Hawaii (W - Whiskey)
NYC
- in winter EST (Eastern Standard Time)=UTC-5
- in summer (march/october) EDT=UTC-4
Belgium:
- in winter UTC/GMT+1, called CET (Central European Time)
- in summer UTC/GMT+2 (between end of march / end of october), called DST (Daylight Saving Time), also called CEST (Central European Summer Time)
A convenient time server for human beings is TimeAndDate.com. Other good sources include:
Internet time
Refer also to crypto-timestamping
Calendars
- iCalendar Internet Calendar - not to be confused with Apple's iCal calendar software
- originally IETF's RFC 2445 in 1998
- refined as RFC 5545 in 2009 https://datatracker.ietf.org/doc/html/rfc5545
- refined as RFC 7986 in 2016, adding support for conferencing systems and a new main VCALENDAR object including a calendar name, description and refresh interval
- iCalendar files typically have the file extension ".ical" ".ics" ".ifb" or ".icalendar" with a MIME type of "text/calendar"
- CalDAV (RFC 4791) defines an extension of WebDAV to access, manage and share calendering and scheduling iCalendar information
Some calendar systems send two files as invitation:
- iCal samples and doc
- invite.ics
- BEGIN:VCALENDAR PRODID:-//Google Inc//Google Calendar 70.9054//EN BEGIN:VEVENT DTSTART;TZID=Europe/Brussels:20231205T160000 DTEND;TZID=Europe/Brussels:20231205T163000
- DTSTAMP:20231205T093507Z ORGANIZER;CN=Joost Pinnoy (BE):mailto:joost.pinnoy@pwc.com UID:37v6ur8shordnktkr47719af02@google.com ...
- part 1.3.ics
- BEGIN:VCALENDAR PRODID:-//Google Inc//Google Calendar 70.9054//EN BEGIN:VEVENT DTSTART;TZID=Europe/Brussels:20231205T160000 DTEND;TZID=Europe/Brussels:20231205T163000
- DTSTAMP:20231205T093507Z ORGANIZER;CN=Joost Pinnoy (BE):mailto:joost.pinnoy@pwc.com UID:37v6ur8shordnktkr47719af02@google.com
- Conclusion: contents looks identical
Time and security
Distinction:
- Monotonic (only increasing or decreasing) persistent time: allows a service to be delivered after a power cycle without any remote help.
- Monotonic instance time may be sufficient for connected services that can get an updated time at startup.