How Time Zones Work
The world is divided into 24 primary time zones, each differing from UTC (Coordinated Universal Time) by a whole or half hour. UTC is the global reference point — time zones to the east are ahead (positive offset), zones to the west are behind (negative offset).
UTC+5:30 (India Standard Time/IST) means India is 5 hours 30 minutes ahead of UTC. UTC−5:00 (US Eastern Standard Time in winter) means New York is 5 hours behind UTC. Therefore, IST is 10.5 hours ahead of EST in winter and 9.5 hours ahead when the US observes Daylight Saving Time.
Why 30-minute offsets? Most time zones are offset by whole hours, but some countries chose half-hour or even 15-minute offsets to better align with geographic or political considerations. India uses UTC+5:30, Nepal uses UTC+5:45, and Iran uses UTC+3:30.
Daylight Saving Time (DST): Many countries shift clocks forward by 1 hour in summer and back in autumn to make better use of daylight. This means the offset between India (which does NOT observe DST) and countries that do (US, EU, UK, Australia) changes twice a year.
Key Time Zones for India
India Standard Time (IST, UTC+5:30) is used throughout all of India — unusually, India has a single time zone for a country of its geographic size.
Important offsets from IST:
UTC (GMT): IST − 5:30
London (GMT/BST): IST − 5:30 (winter) / IST − 4:30 (summer, when UK observes BST)
Dubai (UAE, UTC+4): IST − 1:30
Singapore/Malaysia (UTC+8): IST + 2:30
Tokyo (UTC+9): IST + 3:30
Sydney (AEST, UTC+10/11): IST + 4:30 to + 5:30
New York (EST/EDT, UTC−5/−4): IST − 10:30 to − 9:30
Los Angeles (PST/PDT, UTC−8/−7): IST − 13:30 to − 12:30
The IST ↔ EST offset for India-US scheduling:
Standard time (Nov-Mar): IST is 10 hours 30 minutes ahead of EST
Daylight saving (Mar-Nov): IST is 9 hours 30 minutes ahead of EDT
Daylight Saving Time — When and Where
Daylight Saving Time (DST) shifts clocks forward by 1 hour in late spring and back in autumn, effectively moving an hour of morning daylight to evening. This affects scheduling with:
United States and Canada: DST begins second Sunday in March, ends first Sunday in November. EST (UTC−5) becomes EDT (UTC−4).
European Union: DST begins last Sunday in March, ends last Sunday in October. GMT (UTC+0) becomes BST (UTC+1) in UK. CET (UTC+1) becomes CEST (UTC+2) on the continent.
Australia: Southern states observe DST on a southern hemisphere schedule — DST begins in October and ends in March (opposite to northern hemisphere).
Countries that DO NOT observe DST: India, China, Japan, South Korea, UAE, most of Africa, most of Southeast Asia. These countries maintain constant UTC offsets year-round, simplifying scheduling for those within these regions.
The 2-3 week gap between when the US and EU change clocks (they change on different dates) causes temporary changes in the offset between US and European time zones each year.
Scheduling Global Meetings Effectively
Finding a meeting time that works across multiple time zones requires a systematic approach:
Step 1: List all participants' time zones and identify their typical working hours (usually 9 AM-6 PM local).
Step 2: Convert each zone's working hours to a common reference (UTC or your home timezone) and find the overlap.
Step 3: Prefer times near the overlap's centre to avoid one party being at the very start or end of their workday.
Step 4: Note DST transition periods — the offset between zones can change week to week during spring and autumn.
Common global overlap windows:
India-US East: 8:30 AM IST (7 PM previous day EST) or 7 PM IST (8:30 AM EST/9:30 AM EDT)
India-UK: 1:30 PM IST (8 AM GMT/9 AM BST) is the sweet spot
India-Singapore: 10 AM IST (12:30 PM SGT) works well for both
India-Gulf: 11 AM IST (9:30 AM GST) is mid-morning for both