Day counting (numbering) is a useful method to add and subtract dates, and for data storage of dates. Day counters can easily be changed to counters of seconds or other units.
| Gregorian Date | - |
| Julian Date | - |
| eXtended Day Number (XDN) | - |
| Julian Day Number (JDN) | - |
| Rationalized Day Number (RDN) | - |
| Modified Julian Day (MJD) | - |
| Day of Week | - |
| Gregorian Day of Year | - |
| Gregorian Week of Year | - |