Delta T (ΔT) is the difference between Terrestrial Time (TT) and Universal Time (UT1):
ΔT = TT – UT1
MICA determines the delta T value for a specific date and time by linear interpolation between adjacent entries in an internal table of historical and predicted values. The dates of the first predicted delta T values in each version of MICA are given in the following table.
| MICA Version | Date of First Predicted Delta T | |
| Julian Date | Calendar Date | |
| 2.0 | 2453279.50 | Oct. 1, 2004 |
| 2.1 | 2454040.50 | Oct. 31, 2006 |
| 2.2/2.2.1 | 2454863.50 | Feb. 1, 2009 |
| 2.2.2 | 2455745.00 | Jul. 2, 2011 (12:00)* |
If the date of the delta T file used by your copy of MICA matches the latest date in the table above, then you have the most current version. For the Windows edition, the Julian date of first predicted delta T value appears in "About Delta T" under the "Help" menu. For the Macintosh® edition, the calendar date of the first predicted delta T value, or date of delta T update, appears in "About MICA" under the "MICA" menu.
*This delta T file is included with the MICA 2.2.2 update on CD-ROM that is distributed by Willman-Bell, Inc. See the ordering information for further details. Long-term delta T information is also available from USNO's Earth Orientation Department and The Astronomical Almanac Online.
