BST to EST: time zone conversion

BST, British Summer Time, is UTC+1, and EST is UTC−5, which would make 6 hours. But the UK is on BST only from the last Sunday in March to the last Sunday in October, and New York is on EDT, UTC−4, for all of that time. So a time in BST converts to EDT: subtract 5 hours. 14:00 BST is 09:00 EDT.

Through 2026

Jan 1  – Mar 8    GMT → EST    5 hours
Mar 8  – Mar 29   GMT → EDT    4 hours
Mar 29 – Oct 25   BST → EDT    5 hours
Oct 25 – Nov 1    GMT → EDT    4 hours
Nov 1  – Dec 31   GMT → EST    5 hours

The UK changes at 01:00 UTC: on March 29 from 01:00 GMT to 02:00 BST, and on October 25 from 02:00 BST back to 01:00 GMT. The US changes earlier in spring and later in the fall, which is where the two 4-hour stretches come from.

A worked conversion

The tool below starts at 14:00 in London on Wednesday, July 15, 2026:

2026-07-15 14:00 Europe/London (UTC+01:00, BST)
2026-07-15 09:00 America/New_York (UTC-04:00, EDT)

The Bangladesh trap

BST is also Bangladesh Standard Time, UTC+6, and some software reads it that way. Node.js 22 resolves timeZone: 'BST' to Asia/Dhaka:

new Date('2026-07-15T13:00:00Z').toLocaleString('en-GB', { timeZone: 'BST' })
// '15/07/2026, 19:00:00'      Dhaka; Europe/London gives 14:00:00

GNU date with TZ=BST doesn’t know the name and prints UTC, an hour behind London in summer, and Python’s ZoneInfo("BST") raises ZoneInfoNotFoundError. Use Europe/London, which switches between GMT and BST by itself. Typed into the tool below, BST is read as Europe/London, with a note pointing to Asia/Dhaka for Bangladesh.

In code

from datetime import datetime
from zoneinfo import ZoneInfo

datetime(2026, 7, 15, 14, 0, tzinfo=ZoneInfo("Europe/London")).astimezone(ZoneInfo("America/New_York")).isoformat()
# '2026-07-15T09:00:00-04:00'

TZ=America/New_York date -d 'TZ="Europe/London" 2026-07-15 14:00'    # GNU date
# Wed Jul 15 09:00:00 EDT 2026

Meeting window

From 14:00 to 17:00 in London is 09:00 to 12:00 in New York: three shared working hours. In the 4-hour weeks there are four, starting at 13:00 London time, and a weekly call set in London time lands an hour later in New York.