When one’s availability today translates to another’s tomorrow.
Photo by Richard Tao on Unsplash
Premise
A certain appointment scheduling feature allows a user (the host) to make themselves available during certain periods of their day in a week. Another user (the guest) from a different timezone wants to know the hours in their own timezone on which the host user is available. They may want to see these available hours for a date range, say from a Tuesday to a Thursday. Let’s think about the API I/O.
- Input: start date, end date (in the guest’s timezone)
- Output: a list of the appointment time slots that the host is available on (in the guest’s timezone)
Setting up a use-case
The host is in Los Angeles, CA, USA. The guest is in Dhaka, Bangladesh. The appointment duration is 1 hour. Suppose the host makes herself available from 0800 to 1200 (Pacific Standard Time, tz: America/Los_Angeles) on the following dates:
- March 29, 2022
- March 30, 2022
The guest in Dhaka (Asia/Dhaka, UTC+6) wants to know the host’s available hours on the following days of their own calendar:
- March 30, 2022
Assuming Los Angeles is 14 hours behind Dhaka, our guest should see the following appointment slots for March 30, 2022 (UTC+6):
- 0000-0100 (equivalent to 1000-1100 in LA on March 29, 2022)
- 0100-0200 (equivalent to 1100-1200 in LA on March 29, 2022)
- 2200-2300 (equivalent to 0800-0900 in LA on March 30, 2022)
- 2300-2400 (equivalent to 0900-1000 in LA on March 30, 2022)
The formula
The guest sends the following input to the API:
- start_date: March 30, 2022
- end_date: March 30, 2022
The API needs to translate the given date range (in the guest’s timezone) to a date range in the host’s zone. The API subtracts or adds hours to the start_date of the guest depending upon whether the host is behind or ahead of the guest’s timezone. So, to the API, the start date becomes, in this case: March 30, 2022 at 0000 hours – 14 hours = March 29, 2022 at 1000 hours.
For the end_date the formula is slightly different: March 30, 2022 at 0000 hours + 24 hours – 14 hours = March 30, 2022 at 1000 hours.
So, now (in Pacific Standard Time, America/Los_Angeles) all that the API has to do is, find appointment slots between: March 29, 2022 at 1000 hours and March 30, 2022 at 1000 hours.