I have had a play and these appear to be some values you can use to format the date/time however you want it.
Example:
Sunday 18 August 2019 7:35 and 45 seconds pm
| Value | Output | Note |
|---|---|---|
| d | 0 | Day of the week (0-6) |
| dd | Su | Day of the week (name abbreviated 2 chars) |
| ddd | Sun | Day of the week (name abbreviated 3 chars) |
| dddd | Sunday | Day of the week (name full) |
| D | 18 | Date (numeric) |
| Do | 18th | Date (numeric with suffix) |
| DD | 18 | Date (numeric always 2 digits) |
| DDD | 230 | Day of the year |
| w | 34 | Week of the year |
| M | 8 | Month (numeric) |
| MM | 08 | Month (numeric always 2 digits) |
| MMM | Aug | Month (name abbreviated) |
| MMMM | August | Month (name full) |
| Y | 2019 | Year (full) |
| YY | 19 | Year (abbreviated) |
| YYYY | 2019 | Year (full) |
| h | 7 | Hour (12 hour clock) |
| hh | 07 | Hour (12 hour clock always 2 digits) |
| H | 19 | Hour (24 hour clock) |
| HH | 19 | Hour (24 hour clock always 2 digits) |
| m | 35 | Minutes |
| mm | 35 | Minutes (always 2 digits) |
| s | 45 | Seconds |
| ss | 45 | Seconds (always 2 digits) |
| a | pm | am / pm indicator |
So
{{#custom_datetime}}dddd, D. MMMM Y HH:mm U\hr{{/custom_datetime}}
Sunday, 18. August 2019 19:35 Uhr
{{#custom_datetime}}dddd, Do MMMM Y h:mm:ssa{{/custom_datetime}}
Sunday, 18th August 2019 7:35:45pm
It looks like moment.js has a .fromNow(); function but I am not a programmer and so I assume from calebjohn’s comment above it would require a new template function to be created to use it.