A developer scratchpad

Most programming days have a handful of small calculations in them: how long a download takes, what a timestamp in a log means, whether two files hash the same, what a flag mask comes to. They usually go to a search box, a terminal and a browser console in turn. This note keeps them in one place, where the answers stay beside the question.

# Sizes and transfers
4 GB at 50 Mbps                              // 10.67 min
1 TB in GiB                                  // 931.32 GiB
1.5 GiB in MB                                // 1,610.61 MB
100 MB * 3                                   // 300.00 MB

# Timestamps
1700000000 to date in UTC                    // Tuesday, November 14, 2023, 10:13:20 PM
"2026-03-11T12:00:00Z" to timestamp          // 1,773,230,400
(1773230400 - 1700000000) seconds in days    // 847.57 days

# Hashes
sha256("hello")                              // 2cf24dba5fb0a30e26e83b2ac5b9e29e1b161e5c1fa7425e73043362938b9824
crc32("hello")                               // 3610a686

# Bases and bits
0xDEADBEEF                                   // 3,735,928,559
255 as hex                                   // 0xFF
4096 as binary                               // 0b1000000000000
0xFF & 0x0F                                  // 15
1 << 10                                      // 1,024
2^32 - 1                                     // 4,294,967,295

What each part is doing

Sizes. GB and MB are the decimal units a disk is sold in (a thousand to the next), and GiB and MiB the binary ones an operating system often reports (1,024 to the next), which is why a 1 TB disk shows as 931 GiB. A lower case b is bits, so 50 Mbps is megabits a second, and 4 GB at 50 Mbps is the time the transfer takes. See data sizes.

Timestamps. A Unix timestamp counts the seconds since the start of 1970 in UTC, which is how most logs and APIs write a moment. to date turns one into a date, and in UTC shows it in UTC rather than in your own time zone, so it reads the way the log does. to timestamp goes the other way, from a date and time written in the ISO 8601 form (2026-03-11T12:00:00Z, the Z meaning UTC). The difference of two timestamps is a number of seconds, and seconds in days says how long that is.

Hashes. A hash turns any text into a short fixed-length fingerprint: the same text always gives the same hash, and a change of one character gives a different one. sha256 is the one to use for checking a download or a file; crc32 is a quick checksum for spotting accidental corruption, not tampering. See hashing.

Bases and bits. 0x writes a number in hexadecimal and 0b in binary, and as hex and as binary show an answer in them. & keeps the bits two numbers share, which is how a mask picks out a flag, and << shifts the bits left, so 1 << 10 is two to the tenth. See number bases, bitwise operators and bit shifting.

What the note leaves out

A hash of a file needs the file’s contents, and this note has only the text you type into it, so it hashes text. A timestamp in milliseconds (thirteen digits, as JavaScript’s Date.now() gives) is read as milliseconds by to date, so the two kinds of log line both work, but a timestamp with no zone written as text (2026-03-11 12:00) is read in your own zone, which is why the example above writes its Z.